Do you have to permanently attach a muzzle brake for the upper to be post-ban legal?  If so, how hard is it to do that?  Thanks

- Edited for spelling

Why would you want to break your muzzle?  If you don't like it, you can saw it off.  As long as you break your muzzle at a point so that the barrel is still at least 16" long, it should still be legal.z

Sulaco, same thing happened to me and my first post.  It's "brake" not "break."

Now, as to your question.  You can't have a threaded barrel that will support a Flash Suppressor, so;

1)  You have your brake Pinned and welded "permanently" on or;

2) you can use set screw on a crowned (no threads) muzzle or;

3) you use a thread that a flash suppressor cannot be used with.  I forget which company does this, but it's out there.

As far as how hard it is to do?  Have a qualified gunsmith pin and weld the brake on (if you go that route).  If you screw up the muzzle you'll make the rifle very inaccurate.

editd to add:  If your barrel is less than 16 inches long, then Yes, it must be permanantly affixed (pinned and welded) so that your barrel is at least 16 inches long.

If you want to install a Muzzle Brake the best way (ATF Legal) is to use 56% silver solder paste.  Apply it to the threads, index the muzzle brake on the barrel, use MAPP Gas torch to bring to a dull red glow and allow the solder to melt.  I've done this with 3 different FAL's and it works great.  
http://sra-solder.com

Parts number: STL-1205-655, Brazing Paste.

Hope this helps out.

Aif