I believe that what would happen is the compression ratio would increase, not decrease. It is true that some of the volume gained with the bore will become part of the combustion chamber but the vast majority of volume is in the cylinder bore-top to bottom. More cc's per cylinder and very little change in volume of the combustion chamber. Depending on the math and ultimate determination of what the true compression ratio will be and whether or not you can accept the determination in a practical sense--is fuel available for a higher ratio? etc. It may be necessary to take something off the piston top to get the ratio down to where you want it.
In determining the ratio you also have to account for any recess in the piston top. B18/20 and B21 all had flat top pistons (at least in the US market, except for B21FT). I know that may not be so for later motors. -- Dave