OLD Uis will work just fine, but the older ones do not reskin newer assets, is all. There won't be any game stopping problems.
If you want a part reskinned that isn't reskinned in that particular UI you can dwonload a UI that does have it reskinned and combine it, or you can try to do it yourself with the UI art pack.
I believe in science and that science can explain everything,
Because God has made it all work in such a beautiful way.