What, the antenna? RF tuning is a tricky business. I'd not be confident of a plug and play replacement working properly, nor of it being legal (you'd have an uncertified transceiver after such a mod, working in regulated bands).
Plus the phone would at least require different radio firmware, but many would require chips changing as well.
There have historically been a few odd cases where it's been possible to do such things, but they've been ones where the phone hardware supported the band & protocol but it just wasn't enabled in software. In general the answer really is no.