I contacted Apple Support with the same question.
First level support tech told me it "would be fine", should work, "probably be okay" under repeated questions about how certain he was. I asked to be transferred to Pro Apps technical support. This guy understood my dilemma. I cannot, and frankly should not, have to justify paying for Aperture 3.6 at this juncture, considering it will be discontinued. But, I want to be certain that my Aperture 2.X libraries will migrate to Photos. I no longer require the editing power of Aperture, but of course, do not want to lose access to my thousands of photos in Aperture 2.0.
It was as if this tech in Pro, had not been asked this question before. He consulted "senior" support reps and still was not able to definitively say.
1.) Recommendations Apple has: Bite the bullet and pay for Aperture 3.6. Be certain that way.
2.) Join the Public Beta program (new, separate partition) and "try" Photos migration. I can't do this because of hardware constraints. Another external hard drive would cost as much as the software for example.
3.) Take my Aperture libraries on a backup drive to an Apple store and ask them to update them to Aperture 3.6 format. < This alone tells me there is an update process and a difference in the database organization and perhaps foretells that migration from 2.X will not work.
Apple is supposed to get back to me with a definitive answer. I am frankly not surprised as most manufacturers assume users, especially "pro" users will keep up to date and Aperture 2.X is, what, five years old?
Will repost when I hear back from pro support.