You can upgrade any of your exotics with Xur on any given weekend...

Just requires an exotic shard, and you get the new version and have to start from scratch on building the perks.

I didn't know you had to wait until Xur is offering an upgrade for that particular gun on a random weekend.

That would just be wrong.

Like on a weekend where Xur selling stuff I dont want and I have like 21 coins, I can use those to upgrade my fav Exotics instead.

http://attackofthefanboy.com/guides/destiny-how-to-upgrade-exotics-guide/