After a membership plan is purchased the only part of it that can be edited/changed is the photo and the activity perks it can be applied to. You're free to add and remove activities. However, nothing else on the membership plan can be edited.
As an indication that the plan is not editable anymore, when you go to Products> Memberships> Plans> and click on the desired plan you will see a light blue notification at the top of the plan's page stating 'This membership has already been purchased and cannot be edited'.
[Light Blue Notification Ex.]
If you need to edit the plan after it’s been purchased the most effective option is to recreate the membership plan and re-register all members via quick charge. Another option is to have the client re-purchase the membership via your widget.
When you Quick Charge a membership, be mindful that you can register them at $0 to avoid recharging the client. One thing to note is that changing the initial amount only affects the first charge, future charges will be at the set membership price.
Please remember to close the original membership plan and cancel the customer's membership to avoid double charges.