That's great, all the best and hope your mother gets the PR soon.
What IRCC asked me was to resubmit the forms IMM 1344 and IMM 0008 with my mother now as the primary applicant and her marital status changed

Also they cancelled my original Application where my father was mentioned as the primary applicant and opened a new application with my mother then. The submission date was still the same. They did not ask for PCC or medical again since those were already submitted and approved as part of the original application. The processing then moved smoothly with the new application and my mother already got the PR and the card as well.