Greetings,

You should use (4, id) in the write method. To replace the new field value with old one

Ex:

for id in [category_ids]:
    my_obj.write({'category_id': [(4, id)]})

(0, 0, { values }) link to a new record that needs to be created with the given values dictionary
(1, ID, { values }) update the linked record with id = ID (write *values* on it)
(2, ID) remove and delete the linked record with id = ID (calls unlink on ID, that will delete the object completely, and the link to it as well)
(3, ID) cut the link to the linked record with id = ID (delete the relationship between the two objects but does not delete the target object itself)
(4, ID) link to existing record with id = ID (adds a relationship)
(5) unlink all (like using (3,ID) for all linked records)
(6, 0, [IDs]) replace the list of linked IDs (like using (5) then (4,ID) for each ID in the list of IDs)

Upvote if this solution help you.
Thanks
Regards,
Muhammad Ahsan Maqbool