Providing you've an upgrade key the Anytime Upgrade will work fine.
The main advice I'd offer is this:
Make sure you've downloaded and installed the latest updates first, otherwise you'll be in for a fairly lengthy wait while Windows downloads and installs updates before completing the upgrade.
If you upgrade, then install SP1, and then have to re-install for any reason make sure you carry out the upgrade again BEFORE you re-install SP1 or the upgrade will fail and you'll have to uninstall SP1 to carry out the anytime upgrade again.
When you've installed the upgrade, you'll need to activate again.
My advice would be to use the automatic telephone system by launching Start and type slui.exe 4 in the search field then press Enter. Choose your region for the correct telephone number and follow the automated instructions.
I always make a note of the installation ID first, and then write down and keep the activation ID before entering it in the activation ID fields.
That way, if you have to re-activate your upgrade for any reason just enter the activation ID without having to telephone first.
Alternatively, carry out a full image backup.