yep, but they always contained "previous vehicle name here" specialty on them and didn't know how to change that or what game effect that would have since they moved on from that vehicle.
If you click the "current tank" button, it'll show you your options. Most of the time, if you're any significant amount higher than 50%, you can retrain them to just barely over 50% for free, to a higher amount for 20k, or 100% for 200 gold.
The 200g is never ever worth it.
The free is a joke. If you're going to be moving crew from one tank to another, don't bother unless it's worth it (60%+ on the free option).
What you SHOULD do is sell off all the crummy guys in your barracks (when you sell a tank, the crew doesn't get sold). Then buy your new tank. Then take the crew off your old tank. Then assign them to the new tank and pay the 20k/each on the transfered crew members.
Most of the time, in the higher tiers (tiers 1-3 this is pretty much a waste, so don't bother) you'll have your crew to 100% long before you'll have your tank maxed or enough money to buy the next tank you've already unlocked.
A 100% crew member will become an 86% crew member when you move them to the next tank. That's quite a bit better than 50% and 20k doesn't take long. Each % lower than 100 will lower the new tank's rating for that crew member. As an example, 99% may convert to 82% or so. Not sure on the numbers, exactly, but you get the idea.
If you start working on this as early as tier 3, you should have 100% and specialization by the time you reach tier 5. You should be well on your way to 2nd specialization by tier 6. Those specializations, like fire fighting or repair speed, etc., REALLY make a difference.