You need a threshold for which the changes happen at. You need a way of ensuring assets are loaded and unloaded properly (for performance / game optimisation as a game drags on).
Units do not change "magically". Units change because the timeframe of the Age has moved on (by centuries). The sudden-ness of that change being jarring is something I understand, but that doesn't justify misrepresenting what the Age Transition is actually doing. It's not magic; it's not intended as magic. If the communication of this shift is poor, then criticise that! If you oppose the age change entirely regardless of how well it's communicated or grounded in any narrative, then oppose that!
It's easy to come up with ideas (especially high-level ideas). It's harder to implement them. A common assumption is that developers haven't tried X or Y (the amount of times people have asked "didn't they try leader switching" when one of the very first dev blogs on Civ VII literally says yes, they did is a good example of that). What you should do is discard that assumption, and try and really defend / interrogate your own ideas to refine their depth and clarity.
(this is again more of a design discussion, the same direction as I was headed with GeneralZift. The thread moved on a lot, so I didn't end up replying to him, but it was an interesting tangent)