I am assuming this is a bug, since this seems like a big oversight. I saw a patron (of another creator) report their experience when changing their tiers. To quote them:
I switched to a lower tier bc I was broke, and they immediately took away almost all of the patreon bonuses and they would charge you again in order to go back even though it already took my money for that month.
If the patron already paid for the higher tier for that month, then switches to a lower tier, I don’t think it should take away the rewards they paid for.