Hey Charms, not sure you can without a reset, seems like there's a bunch of folks with the same problem, I'm sure you found the same forum as this:
http://answers.microsoft.com/en-us/winphone/forum/wp7-wptips/how-do-i-change-the-windows-live-id-that-i-signed/56f83660-6ea7-4c4b-979b-ebdaa3b24a8c?page=3
Microsoft 'answers' haha, that's a laugh I don't think

weigh up the plus's with a reset and linking your U.S. live id though
one example:
Whilst waiting for my wife to finish shopping (again) in the car today listening to a radio show; Nick Lowe* was playing live in the studio promoting his new album,
I liked what I heard so I got my phone out and looked it up in Marketplace and read the bio and review (can't do this in UK marketplace)
and downloaded the album,
then hooked the phone up to the aux in, and listened to it on our way home.
when at home, plugged the phone in to the laptop, Zune software opened and added it to my collection, and upping the play count......
......well it keeps me amused
* remember 'I Love The Sound Of Breaking Glass' or 'I Knew The Bride When She Used To Rock and Roll'?
...No...?
...I'll get my coat...