Also some media sources are stating that there is some space for an extra 8 VLS. I am guessing here but perhaps the change in main gun allows more room (the Leonardo 127mm has a revolver/drum loading system that takes up more space below decks than the Mk45). As the ship is ASW focused they are still discussing whether to add the 8 now or wait until the first three are built and re-evaluate (True North Strategic Review - Substack)
Have a look at the images contained in the links below:
RIVER CLASS Destroyer
Type 26 Frigate
If you look at the upper deck area around the VLS, you can see T26 has x2 missile silo's, x1 in front of the other, While the RCD only has the AFT set up, so while there's space that used to be a magazine / silo, you can bet your bottom dollar that the RCN / ship designers are already utilising that space for something else. KNOWING what AGEIS is like, it will likely mean that there;s a computer room, or a power distribution room, as the system is power hungry.
From memory the UK orientates its VLS launchers 'differently; from the preferred US layout & it may be possible to do this & 'incorporate' an extra x8 cells. But most missile system require a hefty base / seat to accept the modules & I think that IF you were designing the ship from scratch you could do this. I think it's a bit to late for RCD. Remember the launchers are circa x3 ship decks high(
Mk41 VLS Naval Launcher).
Finally, the launcher modules take up 2/3rds of the width of the ship in the area where the ship is still dealing with the 'bow-flare', as it transitions to the maximum beam of the ship, so by putting in x4 modules, how much actual space would there be for passageways / cable & electrical routing / HVAC / Pipework, noting that Magazine design rules pretty much stop you from running ANYTHING thru the compartment, unless it services the missile silo ?
Batch #2 would need a radical redesign & then questions of following the Arleigh-Burke design (x1 launcher FWd, x1 Aft, with x2 modules in each), which in turn would logically point to losing the Mission bay capability from the design, effectively mean a 'brand new' ship design, ALL at great additional costs...
SMALL incremental changes between batches makes sense, radical redesign kills your overall capital budget / reduces the number of ships you can actually buy...