Please stop this nonsense.
Reusing PrepTests WILL NOT HURT YOU!
In fact, one of the most valuable things you can do is reuse your PrepTests. Do them. Then do them again. Then do them again. It's good. It's called practice, and practice, by definition, requires repetition.
Merriam Webster wrote:prac·tice verb \ˈprak-təs\
2 a : to perform or work at repeatedly so as to become proficient
b : to train by repeated exercises
Ok, sure, your score on a PrepTest that you've seen before will probably be higher than on a "fresh" one. Whoopie do. The point of doing preptests is not to have an "accurate" preptest score, it's to get better at the test. Musicians don't fret about keeping songs fresh--they do them over and over again. Athletes don't fret about running a drill a second or third time--they do them over and over again. Repetition makes them better. Do your LSAT questions over and over again. It will make you better.
I've come to understand LSAT prep as taking place on two levels. There's a basic level where you're learning about conditional reasoning, game setups, question types, premises, conclusions, flaws, and things like that. Those are important for initial improvement on the test, and a good grasp of those concepts is probably good to get you a respectable number of points above the median.
The second level, as I've come to understand it, is almost entirely about pattern recognition. The vast majority of, say, consistent 165+ or 170+ scorers work the test without having to use a whole lot of mental capacity thinking through the logic of arguments or memorizing what they've read in RC. Instead, they see things about the current test that remind them of things they've repeatedly seen in the past. The LSAT is an incredibly consistent test from administration to administration. The structures of the arguments, the games, the passages, and the answer choices have all appeared multiple times on prior tests. "Hey, this is just like the such-and-such game where this particular rule was the key to most of the questions!" That is where you want to be.
Once you've got a good grasp of the basics, I've found that repetition of old questions is the most effective way to begin seeing (and recalling) the patterns in the test--of getting to that higher level. Redoing questions is like like watching a movie a second time. You begin to see things--obvious things--that you totally missed the first or second time through.
That flaw in that argument you just worked? You're going to see it again. So get familiar with it so that you don't waste any time recognizing it. That crazy clue in that game? It's probably been in 3 or 4 other games. Like the ending in the Sixth Sense, repetition will make these things obvious.
Now, this doesn't mean you should pull out an old LSAT and say, "oh yeah, I remember this one, the answer is C." Knowing the answer doesn't really do much good. Instead, look at the questions with an eye toward articulating exactly what kind of problem it is, what steps you should follow to answer it, what the answer is likely to look like, and exactly why each answer choice is right or wrong. Write your explanation down if you need to. Pretend you're teaching someone how to work the problem. Take people's questions on here and try to explain to them how to solve a problem or set up a game. Post your thinking on questions on here if you're not sure you understand it quite right. Then go through them again.
The usefulness of a question is more than it's ability to simply test whether you can answer it right. But you don't gain a lot of that additional value if you go through the question once, find that you got it right, and then discard it forever. So use your tests. Reuse them. Resuse them again. Abuse them. Wear them out and squeeze every bit of knowledge and understanding you can get out of your materials. It will make you better.