I feel like this is a very bad idea.

Bad idea. Existing deck boards will continue to warp & cup. Rotting will accelerate at moisture holds between old deck and new accelerating rot.

If it is worth doing, it is worth doing right.

Taken from Timbertech website:

What Not to Do: Install Over Wood Boards

Do not install composite deck boards on top of wood deck boards for the following reasons:

Water will seep in between the two boards and cause the wood deck boards to rot and deteriorate in time.
The weight of composite deck boards on top of the wood deck boards may be too great for the substructure to safely hold.
Installing composite decking over wood boards would raise the deck height by an inch or so, creating a tripping hazard.

If you want to make the shoddy workmanship thread this would be a way to do that