Its not an upgrade. The paraphrasing as above, where one merely requests to be added to the waitlist as one is the holder of a business class fare, is the correct phraseology to use. If you start going down the rabbit hole of referring to it as an "upgrade", the agent might inappropriately try to process it as an 'upgrade' which could have undesirable results.
Of course its perfectly possible that AC sells out the J cabin to revenue passengers or confirmed coupon upgraders/deadheading captains, in which case, its back of the 'bus for you...