I have a question:
what do you think of the English UU?
As it is now, it is more historically correct, but has some problems with upgrades.
Upgrading it to a transport loses its attack and bombard strength, while upgrading to a destroyer loses its transport ability. Between these 2 ways, I think that upgrading to a tranport means a smaller penalty to the player and to AI-controlled England (that may remain without tranports, in the latter case).
Switching back to replacing the frigate is still a possibility, but inaccurate.
Another possibility is removing it from any chain.