I am not Catholic so I do not have any idea why this was a rule.
Why did (and apparently changing) the communion on an empty stomach as sinful?
Because it showed disrespect to the body of Christ to have food in your stomach to mix with Christ. It was also very, very important to catch each and every crumb of the Host . . . that's why a patent (I was an altar boy who held the patent) was placed under the chin of each recipient to catch all particles that dropped from the priest's fingers . . . and ONLY A PRIEST COULD HANDLE THE HOST AND THE PARTICLES . . . NO ONE ELSE . . .
Since Vatican II virtually destroyed all respect for the priest, nuns, and the consecrated host, anyone, including the recipient can handle the host. The host could conceivably be dropped on the floor, picked up, and then eaten. Now, either it was a sin with dire consequences to handle the host or it was not.
In any case, the solemnity and respect for all that was sacred to the church has long since disappeared from most suburban, yuppy, trendy, and non-Bible based Christianity.