If you are not using profit center accounting in OE setup, the product category is used if you have a product category/location record in place. If not, it uses the defaults in OE. If you copied these items to a new location, you could change the GL accounts in the new location to reflect your GL preference, process the credit. If the item is returned to inventory when credited, you could then transfer back to the primary warehouse to be reissued to get the correct accounting for a resale.
If you use profit center in OE setup, the customer types drive the GL accounting in combination w/the categories.
You might also have a miscellaneous item called "returns", specify in the description or comments what is returned & tie item master that to a specific prod cat/loc GL account record.
The allowances account in AR cust type maintainence is used for CR processing when you write off differences between the invoice & the customer payment. OE doesn't use these fields for credits.
I'd be hesitant to change the categories once in OEORDLIN without extensive testing to find out what other reports, etc might be affected by changing a category in the line file so it no longer matches the item master & inventory location records. I've seen several clients down for days when the categories don't match between the master & location records. However, the PC is captured at the point of order entry, so it wouldn't be dynamically updated when item master is changed & the product is already on order. Might be worth the trouble of testing all the reports tied to product category. There are a lot of reports that are tied to product category throughout the distribution files, so you should look at them all before you alter the files externally.