Jesus (as) changed the religion to correct it. It was perfect before but the people themselves changed it. And Muhammad (saws) did not change the words of Jesus (as), according to Muslim belief, it was the later people after Jesus (as) and before Muhammad (saws).
As for the sabbath I am not sure. But Islam is the religion that according to Muslim belief has been since the beginning of time. It was the religion of Adam (as), Noah (as), Abraham (as), Moses (as), David (as), Jesus (as), and Muhammad (saws). Throughout time people were given different laws true, but the laws of today's Sharia are immutable and eternal. The Qur'an is the last revelation.
As for Muhammad (saws) there is no other Prophet after him. This is affirmed in many places. And we know that the Qur'an was not changed like the Bible or Torah allegedly was, because we have original manuscripts that were written not even a hundred years after his death. And those manuscripts were all agreed upon by the Ummah and the Hafiz that they were 100 % correct. Baha'ullah contradicts not just the Sharia, but also the theology of Islam. Topics such as hell is a good example.
It is always when the religion of Islam is changed, that Allah (swt) sends a prophet to correct the people. But with this Islam it is perfected. The Qur'an is not changed from when it was revealed, nor will it ever be changed. So no need for new scripture. There will be reformers that will come and bring us back to true Islam, but they will not create new laws or change theology. They will, like the Jews believe the Messiah will for the Torah, will use the Qur'an and not their own words.