If you raise the price of your membership contract, it will NOT impact those already in possession of the membership. The increased price will only impact new purchases of the membership. Read more about editing memberships here.
If you want existing membership holders to receive the new price, you can manually update their renewal price
Updating the Membership's Renewal Price
Run the Membership Details report and filter for the membership contract to generate a list of customers that need their membership prices adjusted.
Navigate to the customer's membership you'd like to update > click the ellipses (...) > Update Price.
Select Update Price for All Future Payments > type in the updated price and Note.
In some scenarios, you might choose to resell them a new membership. Read here for details.