If same type class no penalty but if you say, take a Lowe, which is heavy, and retrain the crew to medium, light, or TD then they suffer a penalty to their primary training percentage. Lets say you retrain them from Lowe to a medium they go down to 60%, or pay 20k credits per crew man and then they are 80% or you can spend gold to make them 100%. But...their secondary skills and perks do not change at all. Only their primary training. The primary training moves along fairly well but when in a gold tank the primary and skills/perks level faster than non gold tanks. Which is why people use them to train up crews. Faster leveling and no loss to skills and perks which are the harder ones to level up. The most you lose is some credits, to train them from 60% to 80% or possibly gold to train them up to 100% right away but most people who have premium accounts dont worry about that. They tend to have the creds or gold to begin with.Isn't there a penalty if the tank type isn't the same, heavy or medium etc.?
You can train even in non-gold tanks of the same nation/type at 25% penalty.