When you submitted the applications for your parents or grandparents immigrate to Canada, If you have specified a Principal Applicant, normally you will not have a chance to replace him/her using another applicant. Actually by now there is still have NOT a change to switch the Principal applicant and accompany applicant. Here we just talk about Replacing, not switching.
From CIC information, the old policy was: PA (Principal Applicant) cannot be alternated after the processing of an application begins. If applicants wish to have their spouse or common-law partners considered as the PA, the original application should be closed and a new application, including new processing fees, should be submitted.
2012 January 17, Canada Citizenship and Immigration Canada Issued Operational Bulletin 369 –
Replacing the Principal Applicant on an Application for Permanent Residence by a Parent or Grandparent sponsored as a member of the Family Class
When the Principal Applicant (PA) on a parent or grandparent application for permanent residence in the Family Class passes away prior to permanent residence being granted and where the sponsor makes a request, the officer may replace the PA with the individual identified on the application as the accompanying spouse or common-law partner in cases where they are also eligible to be sponsored as a member of the Family Class by the sponsor (i.e. not a step-parent or step-grandparent).
Seems the new policy fits for the cases of during the application of parents or grandparents permanent resident with the sponsorship stage passed, and the principal application passed away. We can not see cic listed other cases.
For more detail information, please read cic official site here.