You'll wind up playing your old OFP version, plain and simple. Remember, none of your old OFP versions files have been changed. You'll have access to all your old missions, campaigns and addons. You can even join MP games on servers running the same version of old OFP as you're running.
However, you won't have access to anything from OFP: Resistance. Not the new island, new missions, the new campaign, improved graphics and new sounds.