Are there a risk(s) to downgrading your Minecraft to lets say from 1.0.0 to 1.8.1? Will this mess up your saves or texture packs?
Downgrading carries the exact same set of risks as upgrading — missing/incorrect blocks, terrain generator changes, loss of mod compatibility, etc.
In this case you are risking errors on maps where you have used the blocks/items that weren't present in 1.8.1. The save file format wasn't changed between the two versions.
In your case, where the server never migrated, there is no danger attached. (You should be wary if you try to load a 1.0.0 single player map with 1.8.1, though.)
A good solution would be to have 2 separate instances of Minecraft. Under Windows, there is a super-easy, general solution how to do that: Install the Sandboxie software. That allows you to run programs in a safe "sandbox" - they can do whatever they like, but won't be able to reach your "real" system. Thus they are unable to destroy your saves or modify anything in any way.
It's really easy to use (you just run the program). I'm using it with Minecraft since the release of 1.0.0 with no problems. And the upside is, this solution isn't MC-specific. You can use Sandboxie when experimenting with unknown software, visiting websites you don't completely trust, ...