Don't know how that could be done, to change the unit's appearance all of a sudden when hitting a certain year!
But then again I'm not a digital artist and I don't know how long it would take to make my own textures. I'd love to though.
My all time favorite retexture mod was
Sir Caesar's Retextures, they basically kept the vanilla line infantry model and style but he gave an amazing face-lift (quite literal), with basically, in my opinion, the perfect 18th century general representation of British line infantry (he doesn't have an actual picture of it in his thread, but he does have one of the US line infantry, British look just like those only w/ reverse colors.
However, all his work, save for the very few scraps he recovered and reposted, got deleted. The only way for me to get these textures is through Darthmod (which I hate to death for many lengthy reasons I will not post here) and Minor Factions Revenge. In MFR, I actually found the retexture file and applied it separately via modmanager, but it came with a post-processing tweak that kills my frame rate and sometimes absolutely destroys my visibility. If only I could find a way to extract Sir Caesar's drop-dead gorgeous textures from the IT_Retexture.pack file, but alas I am clueless