How do I fix my vintage world?
Solution 1:
The world generation has radically changed since 1.8 (and will change again in 1.9+), so it's most likely not possible to directly run your world again. In my tests, a world imported from 1.7.3 into 1.8 did have significant changes in the biomes and the landscape.
A workaround which would always work, if you're ready to relinquish new features, would be to keep the binaries (~/.minecraft/bin) of the old version around and swap them around as needed.