Guidance on those types of situations would likely be written into the new order outlining the changes. However, I would be surprised if HPSP players were not grandfathered in under the current system (which I don't see changing for a while, if ever).
The military struggles with retention, especially in the low density fields (such as health care), so removing the pension system would likely cause an exodus of dentists (even more so than the branches currently experience). The pension system works well with obligated service because if you're on the hook for X years, and then you're only Y years away from a pension after that obligation, it's a lot easier to ride it out than get out and get nothing. You see it a lot with Navy/USMC pilots, since they are on the hook for 8 years after they are winged (so about 2-3 years into their career). By the time they can get out, they're over halfway to a pension.
If we go to a 401k style format, then there's no incentive to stick around after your obligated service period. You'll retain the dregs of your manpower pool, along with a few individuals that are truly dedicated to service, but you'll leave a manpower gap that is a mile wide. This is all speculation/personal experience on my part.