Are the bolts screwed straight into the concrete? I'm wondering if the concrete surrounding each bolt will lose its integrity over time after several times of loosening/retightening the anchor bolts. As in: every time the bolt is screwed in, it has to "bite" or "self-tap" into the concrete which chips away a little bit each time. Might be best to leave it as-is. You should also get/make one of those metal beams that connects at the top of each post to lessen the amount of tension being placed on the bolts when a car is lifted up off the ground. Maybe I'm just being paranoid...