In order to transfer benefits, whether or not you are in payback for HPSP, USUHS, residency, fellowship, ASP/ISP/MSP, etc. - it does not matter as long as you have at least 6 years of active duty service, and then agree to serve an additional 4 years from the date you apply to transfer benefits. So, as of today, if you have at least 6 years active duty and are obligated or intend to do another 4 years, get over to CSD/PSD, and get them to do a Page 13 entry for you. (Below is all related to Navy officers, not sure about the other services, but the basic rules should be the same).
Bring a copy of NAVADMIN 203/09 since most people will not know what you are talking about.
http://www.npc.navy.mil/NR/rdonlyres/EFC4BB71-A27E-40D9-B129-1689BD6E1877/0/NAV09203.txt.
This document has everything you need to know, particularly below:
7. ALL OFFICERS, OTHER THAN THOSE IN CATEGORIES LISTED IN PART I PARAS
3C, AND SELRES ENLISTED MEMBERS: THE ADMINISTRATIVE OFFICER WILL
COMPLETE A PAGE 13 IN THE NAVY STANDARD INTEGRATED PERSONNEL SYSTEM
(NSIPS) ESR AGREEING TO SERVE THE REQUIRED ADDITIONAL YEARS OF SERVICE.
PAGE 13 WILL READ: "I UNDERSTAND BY SIGNING THIS PAGE 13, I AGREE TO
COMPLETE FOUR MORE YEARS IN THE ARMED FORCES (ACTIVE OR SELECTED
RESERVE) FROM THE DATE I REQUEST TRANSFERABILITY OF POST 9-11, REAP OR
MGIB-SR EDUCATION BENEFITS TO MY DEPENDENTS/FAMILY MEMBERS. I
UNDERSTAND THAT FAILURE TO COMPLETE THIS FOUR YEAR OBLIGATION MAY LEAD
TO AN OVERPAYMENT BY THE DEPARTMENT OF VETERAN'S AFFAIRS THAT MAY BE
RECOUPED FOR ANY PAYMENTS MADE TO DEPENDENTS/ FAMILY MEMBERS." THE
PAGE 13 WILL BE SIGNED BY MEMBER, WITNESSED AND DATED.
I just recently did this and had no problems once I figured out the process. It was updated in my ESR the next day. Previously, I had tried to apply for transferability, but was denied until I had the page 13 entry done at PSD and entered into my electronic service record. You can see your ESR at
https://nsips.nmci.navy.mil. It has to be done at a DoD computer. If you haven't seen your ESR it's definitely worth signing in and checking out.
Then, go to
http://www.defense.gov/home/features/2009/0409_gibill/. Go to the 'transferability application' on the right side and log in with either your CAC card or myPay data. Your dependents in DEERS should show up. It's easy from here. Click the buttons as appropriate and submit. As long as your page 13 update is in your ESR, you should be golden. You might want to give all of your dependents at least one month, because after you are out of the service, you can no longer add any additional dependents. As long as you are on active duty you can add or delete dependents, from what I understand. You get 36 months to divvy up, but which dependent gets how many months isn't final until well down the road.
Serve four more years, and then navigate the VA system for when you actually want to have your dependents use your benefits (I haven't looked into this yet).
Best of luck.