https://ec.militarytimes.com/static/pdfs/Retirement-Reform-Info-Paper.pdf
According to the DoD white paper on the proposed retirement plan, "All members serving on the day before the effective date of the new retirement system will be 'grandfathered' into the current system, but will also have the opportunity to “opt-in” to the new, blended retirement system. All service members must opt-in within 2 years of the effective date of the new system." This sounds like it applies to all service members, regardless if they're active or reserve, just as long as they're in the service before 1 Jan 18.
Of course, the defense bills aren't final yet and the white paper is only DoD's own recommendations at this point, but the direction this new retirement system is going has been getting clearer this month. We'll see how everything unfolds.