Can't find it written down now, but I know personally last season this was the case for two people I know. They had purchased Cityzen Matchday Memberships in summer only and a few weeks later when they got Season Tickets in that window that opened for Silver only for about a half hour ... their Cityzens membership transactions were reversed a few week or months later automatically.
Technically all Seasoncard holders are Cityzen Matchday Members by default. They get the same 10% off and £5 off home tickets etc. etc. so levels are Non-Paying Member Only (no benefits, just a supporter no. and can build history), Cityzen Members (non-matchday, so pack and discount in shop only) then Cityzen Matchday Member and then Seasoncard holders ... in the system each account can only be ticked as one type, so if you have a ST you can't hold on of the memberships either ... so one each basically. That's why it automatically gets processed. Not sure what length as Cityzenship memberships are rolling 12 month subscriptions so I don't think if you have 2 months left out of 12 if that one would get refunded too, but 1-2 months old definitely have happened.