I’ll have 9.5 years active service when I ETS. Can I keep the legacy retirement if I get out and come back in as a dentist?

If you re-enter in 2018, you have until the end of 2018 to decide whether to opt-in. If you re-enter after 2018 you have a 30-day window to decide which retirement you want.

In other words, yes, you get to keep the legacy retirement system.

http://militarypay.defense.gov/Portals/3/Documents/BlendedRetirementDocuments/BRS%20Frequently%20Asked%20Questions%205.01.2017.pdf?ver=2017-05-02-095830-163

I would think your retirement type is based on your DIEMS date and you would retire under the plan you qualify for now. I had three different active duty periods spread over a 23-year period and each time I came back on active duty my retirement was tied to my DIEMS date, Final Pay and indeed after 38-years I retired last summer under the Final Pay plan.

Good luck with your future endeavors.