The funding issue has come up a lot recently. A lot of people don't fully understand it, but this is what it boils down to in your case:
Residencies get some proportion of the cost of having you on board reimbursed to them by Medicare. If you are in your first residency, they get the full amount. If you decide to switch residencies, Medicare fully funds your second program only until THE AMOUNT OF TIME YOU WOULD HAVE SPENT IN YOUR FIRST PROGRAM has elapsed. That is to say, if you graduate med school and get into a 3-year medicine residency but leave after two years to do something else, your second program will get only ONE YEAR fully funded (2 years in medicine + 1 year in nucs = the 3 yrs you would have spent in medicine). If you initially signed on to a 5-year surgery residency program, you get 5 years fully funded, i.e. you could leave surgery after two years and do a three-year family medicine residency and still be fully funded throughout the whole thing.
Not being fully funded is not as bad as it sounds. It doesn't mean your second program gets NO money for your salary and benefits after your initial period has elapsed. The reality is that the bulk of the cost to your program is still funded by Medicare. The actual breakdown is quite complex and outside the scope of this post, but basically: between one quarter to one third of the cost is the responsibility of the porogram, and two thirds to three quarters are still paid by Medicare. (There are direct Medicare expenditures [DMEs] and indirect Medicare expenditures [IMEs] and only the DME reimbursements, which include your salary, are affected by these rules; DME reimbursements fall to one half of the previous level - not zero - after your initial residency period has elapsed, and stays at that level for the rest of your training, no matter how many residencies you do.)
Whether this would be a deal-breaker depends largely on the institution and specialty. If it's a well-funded academic center, the extra $10-15,000 may not matter much. Also, if the specialty is not that competitive, like Nucs, or the place has trouble filling its spots, or if they just plain fall in love with you as a candidate (or if the PD is your uncle), it shouldn't be an issue. For a smaller community center and/or a residency like radiology, it could unfortunately make the difference.
So, in brief, your NM residency program will get one year fully funded and one year (if it's a two-year program) at partial reimbursement. Your final IM year will be at partial reimbursement. For Nucs, they shouldn't care that much. For your IM year, you could get lucky and find a place with an embarrassing and costly unexpected vacancy (remember, they get amounts of money depending on the number of residents they have on board, among other things, and a vacancy is a liability). IM programs are legion and it shouldn't be too hard to find one, unless you've really got your heart set on the Brigham. Just be aware that some states have a requirement that at least the final two years of residency be spent at the same institution in order to qualify for board certification. Do yourself a favor and look into this.
My advice, like Hans19, would be to finish your IM residency if you can and then go to Nucs, if you want to. There will still be slots open next year, and you probably won't run into funding issues at NM programs. Don't rush into anything.
Good luck.
ariwax