Don has a good idea there. Only negative would be that they will rattle if shaken or blown.
If you cut new panels of anything, cut them a little less than the nominal 2x4'. Normal ceiling panels are 1/4" less each direction, but I find them easier to install if I cut another 1/4" off the length.
If you really want to use the drywall idea, consider 1/4" drywall; it will be half the weight.
But I question the idea of replacing the regular panels. They are not terribly expensive, are easily replaced, and can be painted if looking dingy.