Hummm, trying to play as authentic 1980s Russians, well Ukrainians in my case, I damn thee western states for buying up all the yak-42's!!!
You would buy a new aircraft however when you transfer routes over to it, the arrival times will be different, which means you will probably have to do some adjusting. Do it asap while there are slots available. As later in the game it becomes harder to do.
That being said, while the USSR is still intact I think there should be a ban on Soviet aircraft being bought by NATO nations and vice-versa. Off set some of the extreme cost by giving Soviet airlines some government cheese.