If I have a superior army unit researched to another nation, and then buy a unit from another nation with lower research level than myself, how do I get the unit up to my nations level of research?

I've researched upgraded versions of the Galley since buying these units however they still remain at the same level, also the nation that I bought them from has also upgraded their Galley but the once I had bought off them previously remain un-effected.

Bug?