Why would you bother?
The BluePills are at least twice as fast and the populated boards cost around EUR 1.50.
https://www.aliexpress.com/wholesale?SearchText=stm32f103c8t6The processor itself is a bit old fashioned and has some quirks, but because it is already supported by arduino, so you do not have to worry about it.
More info on:
https://www.stm32duino.com/And while you are upgrading, give Platrofmio a try. It is a huge improvement over the arduino ide.
I've used AVR's for hobby projects since almost the beginning (way before arduino existed), and it was a very nice processor for it's time.
Recently I wanted some more and did some research, and it is not even a contest. Just toss those Xmega's in the bin and use one (or several) of the many ARM processors. The Xmega's are trying to squeeze in a to small niche to be taken as a serious product.
On itself they are capable processors, no doubt here, but that's not the point i'm trying to get trhoug.
You will very likely get some responses under here from people who have used and like these processors.
It's human nature & basic psychology. We defend what we know and are used to.