Upgrading 7 home premium trial to 7 professional

I'm planning to buy windows 7 professional but it's pricy, the upgrade version is much cheaper so I'm planning to install a trial version of windows 7 home premium then upgrade it to windows 7 professional with activation keys. I could save 80 bucks if that works.

Will it work?

Yes, that's pretty nice idea

I don't believe you could upgrade from a trial version. You might need to full version in order to upgrade to best of my knowledge.