Looking at the layout and size of stands in LBA (and without diving into too much detail), there are a couple of stands that are not capable of taking anything bigger than a 737-300. This includes stands 1,5,6 (I know not all these are in use) and also includes the three multi-use stands at the eastern end of the main apron (stands 15-18) which at the same time can accommodate 1x -800 and 3x -300s or otherwise only 3x -800s.
Given that all the stands are taken overnight currently, if Jet2 were to upgrade all the -300s to -800s then that’s at least two less stands available to them. This would mean they’d require both the new stands which are planned to be built this winter and would leave the airport with no room for further expansion Jet2 or Ryanair or a new based airline).
I’m guessing Jet2 are aware that they’ll need to keep the -300s for as long as it takes for even more new stands to be built, or else reduce the based number of aircraft, or request use of the new stands?