Originally Posted by
dwmckee
But there is fair chance the 42 tooth will not work with current derailleur...
Better than fair. Max low sprocket size (as spec'd by Shimano) is 36T. You will see where folks might get an additional 2T out their RD, but let's be realistic, at that point you're running the RD beyond spec and performance MAY be affected, as in won't shift reliably onto the low (biggest) sprocket.
IMO, without scrapping the existing drivetrain and converting to 11 speed (or even 12 speed) and 1x up front, the best option for the OP is to get the 11-36 cassette. It does increase the total gear range a modest 28% from 474% to 502% and it will drop the ratio from 0.882 to 0.833, again modest providing an addition 1.3 gear inches, but it is something.