TRVs and Super Visas are different visas.
Your parents were authorized to stay for 6 months on their TRVs.
To extend their visitor status (ie to extend their period of authorized stay), they can apply for an extension to their visitor status to CPC-Vegreville.
If successful, they will be allowed to remain in Canada for another 1 year.
During this one year, if they leave Canada (for example, for their home country) then this visitor status/extension of authorized stay will lapse. When they return to Canada, they may be allowed back in using their valid TRVs (it's like starting afresh)..... the duration of their new authorized stay will depend on the officer they meet at the port of entry each time.
On the other hand, Super visas would allow your parents to stay for up to 2 years (upon initial entry) without applying for any extension to their visitor status after their initial 6 months stay (which is the case for TRVs).
You can either apply for an extension to their authorized stay now (30 days or so before their current stay of 6 months ends)..... or apply from scratch for super visas from their home country