If you want to edit a product and it has been purchased at least once you'll see the following message at the top of the product dialog:

This indicates that certain properties of the product have been locked to avoid accidental editing. If your site has been running for a while and this product has been purchased numerous times it's usually best not to edit these locked properties. In this case, it's best to create a new product with the new configuration and use it in addition to the current product. However, if you're just working on building your site or not a lot of customers have purchased the product then you can click to unlock the properties and edit them directly.

IMPORTANT: When you edit a product that has already been purchased, the changes will only effect new customers who buy the product going forward. Existing customers who have already purchased the product will be unaffected by the changes.

If you want to change the price that existing customers are paying for a membership, you will have to create a new product and they will have to make a new purchase. There are two ways that you can handle this:

1. Create a new product and membership level (Recommended) - You can send members a purchase link for the alternate priced product and associate this with a new membership level so they can seamlessly upgrade. The upside to this is that they are pro-rated the amount left on their billing cycle and their old subscription is automatically canceled. 

2. Cancel account and then reactivate - You can cancel your member's account from the Access Rights tab and they will move into pending cancellation, during which time they will still have access. Then you can either reactivate their account immediately using the new product or you can set up an internal reminder right before their hard cancellation date to reactivate them with the new product. The difference between these two would be a change in billing dates. The benefit of this process is that there is no action necessary by your members.