Best approach is to Copy your existing plan. Then disable new subscriptions, upgrades, and renewals for your existing plan. Now edit your copied plan and change its price. Then enable your new copy plan. This way existing users continue with their current access, but can't renew and would have to agree to the new pricing (manually).
Another option is to edit the existing plan, change the price, renewals will invalidate and fail and they'll have to re-subscribe with the new price. Either way is really up to you.
Just keep in mind recurring subscriptions will fail and have to be re-subscribed upon time of renewal. There's zero cause for concern for any single payments with either method.