Artnum can not be changed. It's specific to a few details about what is being purchased. Specifically the following

(TYPE)(TYPE ID)-(USER ID)-(SUBSCRIPTION/PLAN ID)

For a merchandise plan you may have something like the below.

M1-42-1

The above describes merchandise plan 1 being purchased for user 42 for plan 1. For a subscription this is a bit different since instead of the 3rd set being plan id it's typically subscription id. Example as follows.

S1-42-3

The above is subscription plan 1 for user 42 with subscription 3. If it's an upgrade subscription it can also have -UPGD on the end.