The T-80UD aren't really all that much newer even. The T-64B was produced by KMDB until 1987, the T-80UDs Ukraine uses are from 1990/91 essentially.
my dilemma would come in terms of the main gun, the ammunition
The T-64B and the BM Bulat use the exact same main gun and ammunition as the T-80UD (or any T-72 for that matter) - a 125mm 2A46M, or the local copy KBA3.
The data given by KMDB also suggests both can fire the same gun-launched ATGM, with an equal (or identical) control system. The autoloader is also identical (6ETs11/6ETs15?).
Of the locally produced guns, KBA3, as i understand it, is simply a 2A46 variant with a trunnion for mounting in the T-64/T-80 turret. KBM1M, for example, would be the same gun with a trunnion for the T-72, and KBA3K would be for T-55 turrets.