As said by another, the concept of rebirth or reincarnation is often different in Buddhism compared to Hinduism.
The rebirth doctrine in Buddhism, sometimes referred to as reincarnation or metempsychosis, asserts that rebirth does not necessarily take place as another human being, but as an existence in one of the six Gati (realms) called Bhavachakra.
Rebirth (Buddhism) - Wikipedia
So we have progress through different spiritual realms after this life which is similar to the Baha’i view of progress through the worlds of God.
But many Christians used to also believe in purgatory.
Purgatory - Wikipedia
So perhaps Christian concepts of heaven and hell have changed with time.
Reincarnation throughout multiple lives while held by a minority of Buddhists are probably held by a majority of Hindus.
For the Baha’i worldview to be proved contradictory you would need to first establish what Krishna and Buddha originally taught. That can not be done as all records of what they taught must be considered unreliable.
I’m not. The two traditions of Buddhism and Christianity originated 500 years apart and thousands of miles away from each other in very different settings. They have evolved over the centuries largely independent of each other. Where they appear to converge is their application to our shared humanity and what works in communities. Neither have properly adapted to the modern world.
We can’t actually establish a fully authentic original form for either religion. The problem is simply compounded by several orders of magnitude in Buddhism as the first Sutras and Sutas weren’t written down for nearly 400 years after the Buddha passed away.
It may not be that different except for the matter of the existence of God which of course is the biggest difference between Christianity and Buddhism.
It’s a plausible and attractive theory.
Beliefs certainly change over time as well as perceptions as to what a Teacher may or may not have said over two thousand years ago.