Ok, So I stated what Paul said about what is not done in Faith is Sin, because the two specific reasons he gave, as I am certain you are aware were, eating and Certain days that some observe and that some don't. I already know your answer but maybe for the sake of others, Romans 14 verse 5 and 6 deal specifically with a Day, you will of course say it is not referring to the 7th sabbath day of the week according to the 4th commandment but that is your interpretation not mine and millions of Christians. Also, Colossians says very much the same thing, also below
Romans 14
1. Him that is weak in the faith receive ye, but not to doubtful disputations
2 For one believeth that he may eat all things: another, who is weak, eateth herbs.
3 Let not him that eateth despise him that eateth not; and let not him which eateth not judge him that eateth: for God hath received him.
4 Who art thou that judgest another man's servant? to his own master he standeth or falleth. Yea, he shall be holden up: for God is able to make him stand.
5 One man esteemeth one day above another: another esteemeth every day alike. Let every man be fully persuaded in his own mind.
6 He that regardeth the day, regardeth it unto the Lord; and he that regardeth not the day, to the Lord he doth not regard it. He that eateth, eateth to the Lord, for he giveth God thanks; and he that eateth not, to the Lord he eateth not, and giveth God thanks.
7 For none of us liveth to himself, and no man dieth to himself.
8 For whether we live, we live unto the Lord; and whether we die, we die unto the Lord: whether we live therefore, or die, we are the Lord's.
9 For to this end Christ both died, and rose, and revived, that he might be Lord both of the dead and living.
10 But why dost thou judge thy brother? or why dost thou set at nought thy brother? for we shall all stand before the judgment seat of Christ.
And at Verse 17 thru 23 the emphasis is not outwardly things but of the heart.
17 For the kingdom of God is not meat and drink; but righteousness, and peace, and joy in the Holy Ghost.
18 For he that in these things serveth Christ is acceptable to God, and approved of men.
19 Let us therefore follow after the things which make for peace, and things wherewith one may edify another.
20 For meat destroy not the work of God. All things indeed are pure; but it is evil for that man who eateth with offence.
21 It is good neither to eat flesh, nor to drink wine, nor any thing whereby thy brother stumbleth, or is offended, or is made weak.
22 Hast thou faith? have it to thyself before God. Happy is he that condemneth not himself in that thing which he alloweth.
23 And he that doubteth is damned if he eat, because he eateth not of faith: for whatsoever is not of faith is sin.
Clossians 2:13-17
13 And you, being dead in your sins and the uncircumcision of your flesh, hath he quickened together with him, having forgiven you all trespasses;
14 Blotting out the handwriting of ordinances that was against us, which was contrary to us, and took it out of the way, nailing it to his cross;
15 And having spoiled principalities and powers, he made a shew of them openly, triumphing over them in it.
16 Let no man therefore judge you in meat, or in drink, or in respect of an holyday, or of the new moon, or of the sabbath days:
17 Which are a shadow of things to come; but the body is of Christ.
We are no longer bound by Specific Days including the 7th Sabbath day. You argue it does not the mean 7th Sabbath day, but I sincerely disagree. And Hebrews 3 and 4 as you have also taken and given a general vague meaning about what it means, is the VERY SCRIPTURE that identifies the New Covenant SABBATH DAY. And it is very specifically addressing Faith ( Belief ) in Jesus Christ as the eternal Sacrifice for our Sins. The Israelites were brought into the conversation as a means to show that we too will NOT ENTER God's REST if we DO NOT BELIEVE and Obey God's Word "TODAY". And what day is He emphasizing? Is it the seventh Sabbath that we are told we need to find our Rest? NO, it is in the finished work of Jesus Christ at the Cross.
Jesus said before he gave up the Ghost "IT IS FINISHED". And we know that what he established (finished) was the means by which we are restored to fellowship and peace with God. WE NOW REST IN THE FINISHED WORK OF CHRIST. Jesus is our REST, not just for a (Sabbath day) but every single day. TODAY If you will hear his voice and believe in Jesus our High Priest, and the one time sacrifice for all sin , we shall cease from our own WORKS ( Trying to earn our righteousness with God which is hopeless). And we can enter into God's REST. Therefore every single day we LABOR, we diligently seek to live in the finished work of Jesus Christ. We know this by actually living and experiencing the Righteousness, peace and joy of the Holy Spirit, not just for one day but every "DAY"
Hebrews 4
1 Let us therefore fear, lest, a promise being left us of entering into his (JESUS) rest, any of you should seem to come short of it.
2 For unto us was the gospel preached, (And What is the gospel other than Faith in Jesus Christ for the forgiveness of Sins) as well as unto them: but the word preached did not profit them, not being mixed with faith in them that heard it.
3 For we which have believed do enter into rest, as he said, As I have sworn in my wrath, if they shall enter into my rest: although the works were finished from the foundation of the world. (When do we enter into this rest? It is the moment, The Day we believe in the gospel of JC)
4 For he spake in a certain place of the seventh day on this wise, And God did rest the seventh day from all his works.
5 And in this place again, If they shall enter into my rest.
6 Seeing therefore it remaineth that some must enter therein, and they to whom it was first preached entered not in because of unbelief:
7 Again, he limiteth a certain day, saying in David, Today, after so long a time; as it is said, Today if ye will hear his voice, harden not your hearts.
8 For if Joshua had given them rest, then would he not afterward have spoken of another day.
9 There remaineth therefore a rest to the people of God.
10 For he that is entered into his rest, he also hath ceased from his own works, as God did from his.
11 Let us labour therefore to enter into that rest, lest any man fall after the same example of unbelief.
So my answer to your question "where is the Sabbath day changed in Scripture". It is in Chapter 4 of Hebrews, but it was never changed to only Sunday, and we are no longer to live only one rest day out the week but Every SINGLE DAY. There is true freedom in serving and living and resting in the finished works of Christ to where we no longer strive to live for and obey the Commandments of God in our own strength, but we cease from our Own Works and live by the Power of the Holy Spirit living in us, made available thru the sacrifice of the Lord Jesus Christ, putting all of our hope and trust in HIM.