Lateral transfers are available, but difficult depending on what everyone else is doing in all the communities involved. If you are trying to get into a fully manned community from any other your request will most likely get denied. Also, if you are trying to get out of an not-fully-manned community they will most likely not release you.
Also, your minimum initial service obligation IS NOT the same thing as having your community obligation complete. For example, Submarine Officer is a 5 year MISO, but your community obligation is not complete until AFTER you Department Head SEA tour, roughly 9 year point.
No one official will say this, but the Navy does NOT want you to laterally transfer. They want you to either stay in your current community or get the F out, especially when nukes are involved. For the most part, there are already people serving in whatever community your are trying to get into so you are not needed.