Here's the skinny:
A prelim year is not considered a terminal residency. The meter only starts to run once you start a terminal residency (i.e. IM, GS). Thus, doing a prelim year and then eventually getting into another field doesn't shaft your program on funding. Programs mainly get killed when someone goes from a short residency and then wants to do a longer one (i.e. IM and change to GS).
I think the above is not correct. I can't speak for osteopathic programs (? perhaps the rules are different) but here's how it works for allopathic programs:
If you match/scramble into a prelim year alone, you are not in a terminal residency. Hence, the total amount of training time that CMS will pay for your is not yet set.
Let's assume you were to do the same thing in an allo program -- repeat a prelim year, then start rads. When you start your Rad training, CMS would limit your funding to the minimum amount of time it would take to complete a rads residency (5 years total = 4 Rads + 1 prelim). However, you've already used 1 extra year, so when you are in your last year of rads you don't get full funding.
So, getting only a prelim alone doesn't limit your funding in any way, but it does use one year of training time. if you end up repeating a year for any reason, you're short. The meter starts when you start any training, its just that the total time/funding alloted doesn't attach until you start a terminal training program.
However, remember that just because you're beyond the CMS training limit, your program doesn't get zero funding. Funding from medicare for resident education comes as two separate payments, DME and IME. For residents beyond their initial training time, medicare pays 50% DME and 100% IME. The IME is often the larger amount of money, so many programs may not care. All fellowships are funded this way, so programs do it all the time. However, given two similar candidates one with full funding and one without, they could certainly favor the latter.