It could be, for experience of the new unit, and the amount of building turns (as percentage of new unit):
New Experience = (Original Size/New Size) x Old Experience
Training Time = (Original Size/New Size) x Normal Training Time for new unit of bigger size
which would take care of the new units needing training, and also starting with no experience. I don't think they should be also able to upgrade equipment at the same time though.
I would be extremely happy if the devs implemented this at some stage.