You'd use the 32 bit Home Premium installer to run the Repair Install
workaround. You cannot in-place up- or downgrade from 32- to 64-bit or vice versa - it must be a clean install.
Another complication may be that you already have SP1 on the install. In that case you'd have to find a retail installer with SP1 included (slipstreamed DVD will not work) or else uninstall SP1 to do the workaround: uninstall Windows 7 Service Pack 1 (SP1)
There are other reasons the workaround may not work, but most will be tipped off when it scans compatibility at the beginning of the Repair Install or In-Place up/downgrade. Remember to set a restore point before you change the Registry Keys shown below, as they must read exactly HOMEPREMIUM
for the EditionID, and Windows 7 HOMEPREMIUM
for the ProductName.