Have you been around Northern's network? Pretty much every station is at a different level.
Just because historically they all built rather randomly, doesn't prevent them being altered. In most cases they are built of brick or stone and solid, so not a case of rebuilding a flimsy wooden structure
These days there are platform edge sections with the yellow warning bumps already built in to rear edge. As long as platforms are being raised it is easy to install them during non traffic hours (more a case of how many extra bricks/blocks are needed as spacers under them) so platform edge is neat and at correct height.
In some cases might have to shut a platform for a few days (or during a weekend closure that is happening anyway) to do the work. Along with laying surface behind the edge stones. It is not like Northern have lots of heritage buildings with multiple doorways in use at many stations so raising platforms causes problems with building access. In some cases might be easier to extend platforms at same time (and if regularly running 4car, 6car trains at peak hour, it is better if they fit in platform to spread the load along length of train and speed up boarding time (and of of course canopies instead of shelters help with this timekeeping too, otherwise everyone tries to get in one door when raining)
Clearly whilst improving the station, some might get ramps, new lights, new seats or better shelters, but that shouldn't really be part of cost of fixing platform height historic discrepancy. A lot of that would have happened anyway over next 40 years (during life of new train)
Should be doing worse platforms first, probably got 5 years to fix these before first trains enter service. And teams doing these on a rolling basis will soon become experienced and slick (and more cost effective) than doing them piecemeal.