Speaking purely from a passenger point of view, I don't believe for a moment that Arriva/Northern will simply look to change the roles of guards under DOO and leave it at that. Ultimately, companies will always try to seek quick and easy methods of reducing costs, more often than not the first point of call being front line staff. At the moment this seems to be contrary to what Northern are doing. There are more revenue staff at more stations, especially in the morning peaks. And it seems to be having an effect, I've noticed a considerable drop in the number of "pay when challenged" passengers on my morning commute because there are revenue staff at both stations I use en route. But travel outside of the morning peaks, and the story changes rapidly.
This charm offensive also seems to replicate what is happening, or has happened with other parts of the private sector. Banks for example started to employ more and more staff to "meet and greet" customers arriving at their branches. At first they helped direct customers to the shortest queue, or even deal with basic inquiries. But then they start to act as a buffer, trying to direct customers away from the human interface and towards the ATMs or on-line services, slowly chipping away at the footfall and creating a growing case for reducing the number of front-facing staff and even eventually closing branches. And similar models have been used right through the retail sector, especially supermarkets.
So as Northern prepare to add TVMs to the stations that did not previously have them, add ticket barriers to more of the larger stations, and start to move towards DOO a similar pattern emerges. Encourage the customers to buy at TVMs, or even better in advance online. And if they don't, then there is always the guard / revenue officer to help advise passengers on how to avoid needing their services in future. As customer behaviour changes, the case (at least in the eyes of the board) to review the future role of the second member of rail crew grows.
Make no mistake, it may not be Northern policy to reduce staffing levels as a result of some routes going DOO at the moment. But sooner or later it will be.