I was under the impression that Class 156s from EMR were the replacement for the remaining Class 153s at Northern. Irrespective of COVID.
That's my understanding too, I don't believe the last Northern franchise ever had plans to retain any 153 units, shame they didn't go into the 155's to form three car sets. Northern has recieved all the extra stock planned and further units for the Ashington line reopening, alongside that they haven't introduced all the services proposed in the last franchise.
150/155/156's having extended repairs and lack of 195/769 availability is the real problem, the first issue should be resolved once the fleets are all done but the second issue isn't going anywhere.