When did you changed your status ? Is it also possible nowadays? What docs were shown?
"The following persons may apply from within Canada:
Holders of valid work or study permits and their family members;
Holders of temporary resident permits (TRPs) valid for a minimum of six months and their family members;
Refugee claimants and persons subject to an unenforceable removal order;
In-Canada permanent resident applicants and their family members who are members of the following classes, determined eligible for permanent residents (PR) status:
live-in caregiver, spouse or common-law partner, protected persons, and humanitarian and compassionate considerations (H&C);
Persons whose study permits were authorized by a visa office abroad, where the permit was not issued at a port of entry;
Family members of athletes on a Canadian-based team, media representatives, members of the clergy, or military personnel assigned to Canada."