To be honest if Russia wanted to take back the entire eastern block I would not that worried, it was theirs before, it can be again if they want it! It would cost them a great deal of money though! Currently the eastern block costs us a great deal of money!
If the Russians really started moving in a big way then we in the west would have to rebuild our forces in a big way, but while its just eastern block nations which were once under their control, I would not get involved. The risk of some kind of nuclear conflict is just to great.
Someone on here said about Belarus! Belarus is with Russia, as long as the west does not try to topple its pro Russian leader I don't see there will be an issue. I did read the other week that Russia is now going to permanently base a combat wing in Belarus, to kind of counter continued western expansion into their sphere of influence.
I would preferred to keep all of the east as a non aligned, non NATO, (no western bases) buffer zone between the west and Russia. But continued western expansion has threatened Russia in a big way which is what I feared it would do.