Activate Same Game but Different Region on Steam.

That depends. Sometimes (never) they respond you in a day(s), sometimes it takes weeks.

Like the man said, it depends. Although you're not going to simply be waiting for ONE reply from Steam Support. It usually goes something like this.

1.You'll send them a message explaining that you need to remove a game from your account.
2.Wait 1-3 days.
3.They'll reply with a copy paste from the SSA which is probably irrelevant. You copy paste your original request into a new reply.
4.Wait 1-3 days.
5.Someone will actually read part of your request and send you back an explanation that game removal is permanent and are you sure. You will reply yes.
6.Wait 1-5 days.
7.Game will be removed from your account.

You can get lucky and have it all resolved in a couple of days, or you can spend a couple of weeks hoping Steam Support gets outsourced to India so they'll stop being so horrible.

It will simply not work or will it replace the EU version?

I know. But I want to play the game at release.

Perhaps they just bullied you. I do not think that they will be contacted with steam in order to lock your key. Wait for the game's release.