Because Win8.1 Pro is similiar but NOT THE SAME as Win7, but is DRASTICALLY different then Vista or XP. In the latter cases the way programs, user data, etc. is handled is different so YES you would most likely run into many things that would need to be reinstalled, told to remove and BUY a NEWER edition of the software or worse "NOT COMPATIBLE AT ALL" and needs to be removed before the upgrade from Vista/XP can proceed. There would be some but much less issue for Windows 7.
No you don't /NEED/ pro unless you plan to do alot of Remote Desktoping and Encryption of files, it is made for 'PROFESSIONAL' (aka Business) use, where is standard version is the 'CONSUMER' use.