Updating from Vista Home Premium to Ultimate for MUI use?

Anyway, onto the problem.

I'm an English speaker living in Japan. I just bought a new system and mistakingly selected Vista Home Premium rather than Vista Ultimate as my choice of OS. While I do speak Japanese, I'd still rather have my OS in English, and from what I know through my research, I need Ultimate to get the MUI up and running.

Have spoken to the company, and all they can offer is my completely returning my system, them giving me a refund, and me ordering a new system. This is going to cost me a lot of extra money, time and effort, and at this point it seems like it will be way easier and around the same cost to just buy the upgrade from Home Premium to Ultimate on my own directly through Microsoft.

What I want to confirm is, if I upgrade my Vista Home Premium (Japanese version) to Vista Ultimate (still Japanese version), will I then be able to use the English MUI successfully? I read somewhere that an update won't get a MUI working and that I'll need to buy the full version of Ultimate and re-install, but this seems completely ridiculous to me. I don't see why the MUI wouldn't work if I just upgraded to Ultimate.

Still, I wanted to check that I'm doing everything right before going and spending the money on the upgrade.


Thanks so much in advance!