Can I use this plugin to unload a plugin to update then load it? Trying to use to to update Mypets. I did unload, move new jar in and old jar out, did load and its giving me and error while enabling plugin.

Yes, however some plugins don't support reloading in this manner (for example EssentialsX will throw a fit if you reload it).

I've use this plugin constantly to reload my plugins while I'm developing them and have never had any problems with it.