wlewisiii has already commented upthread about transitioning to a new AFV, which goes well beyond simply training the crew. He's a retired tanker from the US Army so he's got hands-on experience with this stuff.
It's not as simple as turning the key and going for a joyride. Russian tanks don't have a loader position, so first you're going to have to expand your armored crews by 25%, and train the news personnel for a new position on a new model of gun. Then you're going to have to train the mechanics who keep it running, and then ensure a supply-chain of entirely new and incompatible parts is in place to keep these tanks running. Mr Lewis covered it very well upthread but this thread is going fast enough that his post is buried in the weeds and I don't have the time to find it.