Credit Note Item Line Fields

The department issuing credits deals with 2 sets of customers, we had previously created a field giving the reason for a credit note which was sourced from a list.

It has recently been brought to our attention that one set of customers requires a much reduced list of credit reasons to the other and the users have asked for the list to be filtered.

I have achieved this by setting up a custom record instead of a list and filtering the options based upon a custom input form for the 2 different sets of users, which seems to work ok. I had to create a new field else we will lose any preexisting data. Unless anyone can suggest a better way of achieving the same result using the existing field?

The one issue I have now is I ideally need to populate the original field (list) with the data from the new custom record based list which is selected by the user. I have tried using sourcing and workflows for this and do not seem to be able to get it to populate with the correct entry, for my workflow I had to use the field name in Formula as it wasn’t selectable, however the value is not the same.  The reason I require this is that we have to keep the existing data, the data feeds into external reporting systems, for the time being I want to avoid updating the reports hence the reason to populate both fields with the same data. Any ideas if I am doing this the best way, or how I might best achieve the desired results?

Either, use exiting field and filter for specific users or

copy value of existing field into new field on item line of the credit note.

Any thoughts appreciated. Thanks