How to reproduce:

• Create a new world in 1.6.4 and find a Taiga biome
• Load world in snapshot
• Observe that it is raining instead of snowing and water is not freezing anymore

What I expected to happen:
Short version: It would stay cold, i.e. continue to snow and freeze water
Long version: Since it was announced that 1.7 would have both cold and warm Taiga variants, I'd have expected the cold variant to take the biome id of the old 1.6 Taiga biome and the warm taiga variant to get a new biome id assigned. That way, Taiga biomes generated in 1.6 would continue to have snow after switching to 1.7 even if the F3 page would now call them "Cold Taiga" instead of just "Taiga".
I really hope they change this for everyone who wants to continue using 1.6 worlds in 1.7 even if it changes the terrain generation...

To test: Generate 1.6.4 world with seed 506555500888585933 then switch
to snapshot. A cold taiga can be found at around x:0 z:580