Change version in 1.8 cause problems

What I expected to happen was playing a version complete

What actually happened was playing a version in alpha/beta

I tried the lastest update of minecraft (1.8) and all seemed works fine but there were some minor problems (graphics gliches) so i decided to restarted playing 1.7.2 version. When my world got reloaded i discovered that my inventory and my chests became empty like a spawn in a new world but with the levels of experience that i had before.(the first time chest was full when i tried another time they became empty)
i'm not happy at all.You must take more attenction in update.

(i'm not english so the text cannot be perfect)

Steps to Reproduce:
1. Update version to 1.8
2. Play an "old" world
3. Exit and change profile to 1.7.2 version
4.Play the same world another time