I want a new upper.

[chiak47]

I bought a XM15 with a 20' barrel.I thought it was a 16' cause thats what it was advertised as.
Anyways I like it except I want a 16' m4 style upper to fit on my bushy lower.

Are they easy to switch out?

Also is there a chance something might not fit?

[imanaknut]

With the exception of some Colts, all uppers should fit any lowers.

[Poodleshooter]

Yes. Push out the rear takedown pin, and either push out or unscrew the front pin depending on what kind of receiver you have. Remove the upper and replace it.
Only older Colt lowers have the wrong hole size (.312"). All post bans are fine and have .250" holes. There are conversion pin kits for the older Colt lowers.