I think Ran is talking about the User ID, not the personnel number. Infotype 0105, subtype 0001 is the "System user name" (SY-UNAME), but the value put in is referred to as "ID/number" in the infotype, which can be confusing if you're not used to it. So what Ran is saying is that contractors are hired as permanent employees, their SAP usernames change along with their employee IDs (or personnel numbers).
I can't speak to how IdM would handle this, as we don't use IdM (yet!) here. In the "old-fashioned" way of doing things, running RHPROFL0 would result in the creation of a new user account.
Regards,
Matt