Unfortunately, the plan or product code cannot be changed. Once a code has been set, it is used as a reference and is locked in in order to prevent breakage elsewhere (such as API implementations of the Fusebill platform).
However, when adding a product to a plan in the catalog, the product code can be replaced with something different. This may cause separate issues around reporting and the API.
The best solution will depend on your business needs and how much the product code has been used already.
The code is not directly surfaced to customers, only internally to the Admin UI and through the API.
If the code is already in use across the account and there are already subscribers to the same plan since the product was added, then it would likely be best to leave it as-is in order to maintain previous reporting. The list of subscribers is available through the plan and the subscription by product report.
To get a plan's current subscriber: Go to catalog->plans->click subscribers on the plan
The next screen should look like the screenshot below. It will list all of the customers currently subscribed to the plan
To get the subscription by product report: Go to reports->subscription by product report
Select the plan, frequency and period. Click generate and then click export
A pop up will appear, click export
The list of subscribers associated with the plan will be available in an excel file.