Is there anyway to downgrade to an earlier version of Sins?

The ancient 1.05 patch might be still downloadable somewhere, you could get 1.05 version by reinstalling sins and applying that patch. All patches after that have been distributed trough Impulse, so you would need to have a backup of Sins with the patch you want to play with, as they are not available anywhere.

Great, new question. What do you have to do to update a mod to 1.17 of Sins, from 1.16?

you can download Harpo9999's mod updater (go to www.sinsofasolarempire.com forums and use the search funtion), although usually the Mod team take it upon themselves to keep mods updated...