For example, when playing Mutual Accord you can attach Spear of the Mark to any gondor hero (non-rohan, e.g. Mablung ). After the phase Mabulung dosen't have the rohan trait anymore, so do I have to discard the attachment?

And a 2nd question which just came to my mind: if I do have a character with two restricted attachments, can I discard one to add another restricted attachment?

1. No, checking requirements only occurs when you're attaching to a character. They keep the attachment.

2. Yes you can.

And just to add: you can't discard an attachment just whenever you want to, but when you play an additional Restricted attachment on a character (beyond 2), you are forced to choose and discard one Restricted attachment from them.

I thought you could get rid if an attachment at anytime.

Nope, only if some forces or allows you to discard it (for example, Miruvor can discard itself any time you have an action window, or a shadow effect might force you to discard an attachment). Same with allies: you can't simply discard an ally, something has to allow it.

You couldn't, for example, discard Expert Treasure Hunter just to ease some attachment hate from a card like Leaves on Tree .

Ahh yes very good point thanks for that input.