One more option: Multi-algorithm.
That is, the dev can introduce one more CPU favorable algorithm (GPU resistant). At the same time, it is not right to change the rule of game so quickly (unless it is an exploit). So we can have two or more algorithms running together. One good example is DGC. Quoted from DGC ANN at bitcointalk:

Algorithm Control - If the last three blocks were discovered by the same algorithm, the fourth block will be rejected until its found by one of the other two algorithms. This prevents directed attacks on the network from any single algorithm."

This may be done by openning one more witness as CPU-mining witness. Or simply allow multi-algorithm to mine on the same witness.