Yep!
At the parks, it's no problem. Over the phone, it's a bit more of a hassle, since they can only use a couple of cards at a time, unless they transfer you to a specific department.
I would just allot some time, and then take your stack to Guest Relations. Actually my favorite plan is to start with a MYW pass and then upgrade it at GR inside the park... seems to be the least hassle/frazzled CMs and the most time efficient. Especially since then you can do it at your leisure!