You should not start charging double to existing customers (which is why Stripe doesn't allow you to edit existing plans).

You should keep them on a grandfathered plan for at least a period of time and/or nag them to upgrade (optionally with a discount for their trouble).

Just keep their current pricing as-is until the next time they need to change plans.

I ask first. If they say "no", I just let them stick with the old price. I may then ask again in 6 months and see if I get a different answer. I also sometime just cancel the billing and let them continue to use the service with the understanding that there will be no support. You gotta experiment.