Why would this be the case? Are you anticipating technical complications, public opposition, etc?
The increase is certainly do-able, though complex in the context of a very active rail corridor.
I expect that yes, there is a material risk of increased opposition, in an area not exactly friendly to the proposal to begin with (above-grade O/L) , to say......whoops, we snuck in as much as an extra 4.5 ft (plus) on the ROW and the Sound Barrier; affecting shadows and visual impacts........
Problematic on its own, worse that it appears as though there may have been an attempt at deception. (quietly making significant changes w/o flagging them).
Added costs are also an issue...........remember the argument that burying this section is an extra 800M.
We could debate both the accuracy of that number; and whether or not 800M would be a reasonable sum in the circumstances......
But it would certainly appear as though cost just went up, materially on this, the 'preferred' option.
So what's the cost difference now? 700M....? 600?
We don't know..........but people will surely be asking.