Hello SpoonsV! you will lose everything associated with your character, the lockboxes or tesseracts (for example) and clones should work after as they are still waiting for and locked to your name.
Count on losing: Enderchest contents, Knapsack contents, inventory and Genes.
Empty the Knapsack and enderchest, take a sample of your blood and drop it in a chest for your return.
You will need to remove the .dat file from where the world folder is located (on the server) />World>players>yourfilehere.dat
I'd suggest simply renaming your file, so if you wish to go backward you can always just erase the new one, and properly name the old one once more.