I've seen a couple people ask about how to downgrade back to 1.7.3 to play on the server and i found that MC Patcher lets you do it.
And there you have it, easy swapping between the new and the old
- "MCPatcher Downgrading":
First, click the circled browse button and it should take you to yout ../.minecraft/bin/ folder where you should see the files shown below (or similar)
Basically, select the 1.7.3 file to load that and then unpatch and repatch your MC and you will be running 1.7.3 again
To set it back to 1.8, simply do the same but with the 1.8 file, simples
And there you have it, easy swapping between the new and the old