It won't mess up your preorder, as that was not made with your Bungie profile. The preorder will have been made with your Blizzard account and as such, whichever account you used to preorder will still hold your order. When merging accounts, currently you can't merge profiles in the traditional way. In order to unlink your Blizzard account from the unwanted profile, you'll have to link another account to that profile first. I'd suggest creating a throwaway XBLA / PSN ID and linking it to that profile. Then log in using that new account and unlinking your Blizzard account. Once you do this you can link your blizzard account to your main Bungie profile. Alternatively, if you don't want to go through this process, you can wait until Bungie reenables the linking process. There is no set time for that to happen yet though.