How would I go about transfering to the USCG from National Guard?

The NG is an odd-duck - as 99% of you is governed by the state rather than Uncle Sugar.

We do have a limit of six years (72-full months) of AD time to be able to enlist in the Coast Guard. This is because you'll lose rank coming over (yeah, it'll hurt), and we still have High Year Tenure to deal with, and we want to make sure you have enough time to make rate so you don't get booted. You'll get bumped back to E-3 or E-2, have to come in as a non-rate (no MOS, like 99% of Coasties), and work at it from there.

Once you get back from Sand-Land, go talk with a Recruiter face-to-face and get the latest info from them. This crap changes by the week - or even more often - so they're the best source of good info.

Master Chief Motyka is spot on with his response but I just wanted to empathize that just because the Coast Guard falls under DHS (law enforcement reasons) everything works very much the same as the DOD branches. As for the switching process, if you’re not at the your ETS then the ANG will have to agree to release you. A CG recruiter can give you guidance on that process. I had 8 years prior service before making the switch: 4 AD years, followed by 1 year in the AF Reserves, and 3 years in the Air Guard. In addition, I was an E-5 before taking a reduction down to E-1 (for basic training only) then advanced to E-3 immediately following basic with a 3-year wait list to Aviation A-school as a non-rate (E3). Ouch! You can beat the High Year Tenure odds if you put a plan in place and follow-through with that plan. The standard (plan) I set for myself was to advance (rank up) at least once at every unit assignment. It works to your advantage if you choose a rate (specialty) that’s fast moving. (Aviation rates are usually one of the slowest.) Do not wait for a policy change before you begin to execute your plan. Policies are implemented much faster in the smaller services leaving many members wondering how they fell into that policy pool. — Semper Paratus

Your Commanding Officer can call the USCG and put it through.