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There is no real mechanism creating a right to switch from active to reserve status. This is because they are technically two different entities that get their money from two different locations. Fiscal law makes them separate.
So, really what has to happen is that you have to be released early from your current active duty tour. Then, in a separate action, you'd have to apply for the reserves and they'd take into consideration your prior service.
To do this, everything will depend on what your current command decides. If they are willing to separate you early, given the situation with your girlfriend, they can certainly choose to do so as a "hardship." That being said, in the Navy people are kept away from their family on a regular basis, so your command certainly might not consider this a true hardship without some other evidence (like evidence of your girl friend having medical issues, or the baby having medical issues, or your girl friend having financial issues that would require your presence). If command is willing to separate you, then it can happen. If they are not, then you cannot leave active duty early in an automatic transfer to the reserves.