I don't think that this change will ever happen. For different PoW algos there is alt-coins like litecoin.
If any of the encryption used is broken or greatly weakened, they will have start the process.
You may be right in that they won't change it due to ASIC being much more efficient than GPU, as they didn't do it when GPU took over CPU. However, CPUs and GPUs are general purpose, commodity products while ASICs are for a specific purpose that only a few would purchase. Everyone else would become marginal and try to have changes made.