There are many scenario where category is allowed to be changed, and many that are not.
Your best bet is to test various scenario - with different approval status of the PO header, line; whether tax is in place, whether the PO is partially or fully received or billed.
If the UI allow changes to be made, then change is allowed and down stream activities should be correct.
In R12, a new profile option is introduced related to change of category.
See this note for more detail:
Item Category In Purchase Order Form Is No Longer Updateable? Does Not Allow Update? Cannot Update the Item Category? Cannot Change Category? (Doc ID 1531948.1)
Hope this helps.