It will create mis-matched boundaries, I've tried to create a map with 1.8, played on it a little and then re-installed 1.7.3 ... The biomes was a mess, no snow but water was turning to ice in normal biome, mountains were cut off abruptly to create new terrain from the 1.7.3 version.
So yeah it will work, but if you create the terrain on 1.7.3 or 1.8, it will stay as it is but new terrain will be generated compared on which version you are using.
See pictures: