Well, that's the list of what I've tried (this world also uses some blocks from my custom mod):
1) 1.6.2 > 1.7.10 without Forge/my mod, vanilla blocks are okay
2) 1.6.2 > 1.7.10 with Forge and without my mod, vanilla blocks are changed to random (air to glass panels, stone to vines, etc.)
3) 1.6.2 > 1.6.4 with Forge and my mod updated to 1.6.4, everthing's fine
4) 1.6.4 > 1.7.10 with Forge and without my mod, vanilla blocks are okay
5) 1.6.4 > 1.7.10 with Forge and my mod updated to 1.7.10, no block conflicts, but air is changed to glass panels, all other vanilla blocks are changed to planks, mod blocks are okay.