I will respond as best I can to your question. First, I know that in the two letters I received after being selected as an alternate, it said I was pre-qualified for service as Reserve or Guard JA. I was and am interested in pursuing this route if Active Duty continues to not work out. However, AD is by far my preference, and I intend to keep applying until family or career circumstances force me to cease doing so.
My understanding of "pre-qualified," based on conversations with a current KY Guard JA, is that it means you do not have to go through the normal Reserve/Guard Accessions Board, but you still have to meet with a Reserve/Guard unit/legal support organization and gain the approval of the SJA or whoever heads the office. Also, at least in the case of the Army, you go through the same training as AD JAs.
As far as going Reserve/Guard and then transitiong AD, I have heard so many conflicting anecdotes that I have no idea what the reality is. My former FSO told me that when he went through JAOBC, a few of the top Reserve/Guard JAs were offered active duty slots at the end of training. I've talked to other people who've been through JAOBC and never heard of anything like that happening. In terms of actually beginning service as Reserve/Guard and then going AD, I've heard/read that if you can do so at all, it's not until after at least four years. Others, including the KY Guard JA I spoke to, told me that going Reserve/Guard is a disadvantage for future AD apps because the Reserve/Guard has an interest in keeping its current JAs. Frankly, I don't know the reality, and am extremely unqualified to speak to this. If anyone more knowledge has better info, please feel free to chime in.
I hope this is a somewhat coherent answer to your questions, and if you're a current applicant, best of luck.