I have two SPG's, Tier 5 and Tier 3; My Tier 3 crew is rather experienced (better than the Tier 5) and I intend to retrain them for my impending Tier 6 SPG.
However; will I still be able to use the crew in my Tier 3 SPG as well?
And will the retrained crew gain experience that they can use in the Tier 6 SPG if I put them back in the Tier 3 SPG after they've been retrained?
Crew not trained for a specific vehicle will suffer a serious performance hit when trying to operate it.
Once you retrain your crew to the tier 6 SPG, they will lose their training qualification for the tier 3 vehicle. You can still use them on it, but they will not perform well on it. (unless you retrain them a second time for the tier 3 vehicle, but then they will perform badly on the tier 6 one.)
You still get experience for operating a vehicle with an unqualified crew, but it will be harder to earn that experience as the crew is not performing as well as a qualified crew would.
It is also worth noting that, whilst crew is indeed only trained to one specific vehicle, If you have a premium tank, the effect of not being trained to it, is negated. Now, in the case of your arty, it won't help as there are no premium arties aside from the LefH which you can't get anymore :P Also, the effect is -20% (if the crew xp % shows in red - meaning not trained to this vehicle.) on the same vehicle type (i.e. spg-spg) and -35% when swapping crews between different vehicle types.
For more information, you might want to take a look at this: http://wiki.worldoftanks.com/Crew#Roles
