Spec question.

[krittical]

Hi my guild is currently working on heroic warmaster i was planning on being affliction but for other fights i do well as demo. Will i still do ok as demo with reforging as Affliction.

[Abiel]

I highly doubt it. Heroic Warmaster requires a decent amount of burst, and playing affliction stacking mastery isn't going to help you with that. If you absolutely can't reforge, stay demo; but I would recommend reforging for this fight and possibly switching to Destro.

[Prag]

There's an addon called ReforgeSaver that lets you save reforges and 1-click swap them. You can have one for Mastery and one for Haste. I'm sure that, for the sake of progression, your guildies wouldn't mind summoning you back.

[OOMM]

Demo is more than fine for warmaster. It's easier to do higher dps as Affliction, but they both have about the same potential. It's going to come down to skill a lot more than spec.
If you reforge to full haste and play demo for other fights it will work out fine. Mastery is better, but haste is pretty good too.