I don't think any changes are necessary. Very soon anybody will have a computer capable of GPU-mining Litecoin. Also, since eventually people will stop using GPUs to mine bitcoin, the swings in difficulty from people switching between the two chains won't be a problem.
Yes, dont threat. GPU's are at most 10x faster than cpu's for litecoin mining. For Bitcoin it is at least 100x faster on a GPU.
This means that Litecoin is at least 10x more efficient on a CPU than Bitcoin.
Over time, this gap will narrow; as the in-built GPU's in inside every computer get better.