How are passengers getting better seats if the only improvement is the seat covers are slightly newer and cleaner?
Do Northern think tweeting repeatedly the picture of the new interior and claiming they are new seats will attract more passengers? If they are then surely that is false advertising. If not why are they are bothering to do it? 99.9% of people won't care what the interior of a train looks like if it isn't being used on the service(s) they will use.
If Northern do replace the seating on the ex-FNW 156s with brand new and upcycle the old seats and put them in 150s which currently have Ashbourne seating they can claim the change in seats is part of modernising the 150s, as the upcycled seats will be newer than what was in them previously. However, with the ex-FNW 150s they've done little more than what the old Northern franchise did to the seats, when old Northern were criticised for not doing enough with their train interiors.
Filling out the form on the ASA site takes 90 seconds. I imagine you've spent more time telling me why I shouldn't bother than I spent bothering!