They could revoke a good will deletion if it was a purely voluntary deletion on their part, meaning it was made without having received anything of value from the consumer in exchange for the deletion. Agreements that dont involve an exchange of something of value are not binding contracts.
If the GW deletion were, for example, part of a pay-for-deletion agreement, then both parties are exchanging something of value.
The creditor or debt collector gets $, and the consumer gets the agreed deletion.
However, if the GW deletion is simply a voluntary "agreement" by the creditor/debt collector to delete, then it is not a binding legal contract and the consumer would not be able to sue for breach of contract should the information later be reinserted.
Can a GW be revoked? Yes, if not part of a PFD agreeement.
Will it? Not likely, since it is a lack of good faith or a poor business practice that might result in a headache on their part, such as answering a BBB complaint that asserts a poor business practice.
I think it's always a best practice to get any agreement to make a GW adjustment in writing from any lender. This way if it ever does pop back up on your credit report, you have the documentation to state that it shouldn't be there. GW approval letters normally don't mention anything regarding an actual GW adjustment. They usually just state something very simple like "...we are submitting a request to the credit reporting agencies for your account to read paid as agreed for the months of Jan-Mar 2015..." So, if something down the line pops back up from Jan-Mar 2015, you have a piece of paper stating that it shouldn't be there. If you were to submit a complaint to the BBB and/or CFPB and upload said letter, I have very little doubt that it would be rectified very quickly.