There is no question of "sell or not" for the crew. After the current tank crew's main skill is 100%, they can continue to train as the next combat team, provided that the next tank car of the current tank is available.

Take KV as an example

1 and KV have completed the research and development of kv-1s. You want to sell kv and buy kv- 1.

2. Move the mouse to each passenger, right-click, find "kv- 1s Retraining" under the profile, and click "Training".

3. After the retraining, right-click one by one and click "Transfer to Barracks".

4. buy kv- 1s without combat team.

5. Click on the kv- 1s passenger vacancy, and right-click "Back to the garage".

Ok, so you have the kv-1S combat team trained by the original KV unit.