Is it possible to convert BAM file from one genome assembly to the other?
It is possible, with tools like CrossMap, but my recommendation is to re-align. Assemblies can change a lot between versions and your reads will undoubdedly align differently. Re-aligning is cheap, it will take a few days and you're good. Re-doing your analysis because you realize the coordinate translation messed something up will cost you much more time and energy.