"This enactment provides that a member’s seat in the House of Commons will be vacated and a by-election called for that seat if the member, having been elected to the House as a member of a political party or as an independent, changes parties or becomes a member of a party, as the case may be."

From 2011. I'm told Poilievre voted against. Regret?

Bill C-306 (Historical) | openparliament.ca
https://openparliament.ca/bills/41-1/C-306/