What happens to existing members if I switch a membership plan off?

No. In Plugins → Membership → Plans, edit the plan and untick Active, then click Save plan. Inactive plans are hidden from the storefront, so nobody new can join, but existing members keep their benefits.

To end someone's membership yourself, go to the Members tab, filter by Status or Customer ID if needed, and click Cancel on their row. They keep access until their Expires date, then it lapses.

You can also untick a single perk's Active box inside the plan, or click Remove next to it, if you only want to change what the tier includes.