I've never had any issue changing from 32 bit to 64 bit. I've rebuilt larger XP machines moving them to Pro without any issue in obtaining the key. Just punch in the old XP serial number and new Pro number and done. Maybe the restriction was a while ago?
In v4 of the agreement it stated that
2.6 Installation of Windows Operating System Software. Refurbisher may only install Windows Operating System Software in accordance with the following table. This section does not apply to Citizenship Licenses.
Windows XP Starter Edition / Windows Vista Starter / Windows 7 Starter / Windows 95/Windows 98/ Windows Millennium (Me) Edition/Windows XP Home Edition/Windows Vista Home Basic/Windows Vista Home Premium/Windows 7 Home Premium/Windows 8/Windows 8.1
that only Windows 7 home premium could be installed as a refurb coa. Pro was not elgible.
Whereas Windows 2000/Windows XP Professional/Windows Vista Business/Windows Vista Ultimate/Windows 7 Professional/Windows 7 Ultimate/Windows 8 Pro/Windows 8.1 Pro
Could be upgraded to either Premium or Pro.
On v5 of the agreement, this table is missing.
I have just emailed the MS refurb dept with the question.
Will post back when I get a reply.
I would say yes, we can upgrade to pro for all licences, but without having it in black and white, I would not stand by my words.. :/