Yes. You can edit both the usage terms (credit type, membership type) and the payment terms (packages and contracts). 

Any changes made to a credit package or membership contract will not impact sales that have occurred. If you change the price, only future sales will have the updated price.

If you change the name of a contract or package, the new name will be reflected on past and future sales. It is recommended that if you wish to change the name of the membership contract, you create an entirely new contract.

Once a membership has been created and sold, any changes you make at the Membership Type level will apply to those who have already purchased the membership as well as those who purchased after the edits were saved.

Please note that if you change a Membership Type from "Members Only" to "Members and Guests" you must immediately edit the Membership Contract(s) to specify the number of guest usages per payment interval as it will default to Unlimited.