Last time, I talked about one way of periodizing Okinawan, based off of non-linguistic criteria. This time, I’m going to talk about two linguistic criteria we might use to differentiate Modern Okinawan from earlier forms of Okinawan.
The first criterion is a major reanalysis in the verbal system which occurred at some point in the history of Okinawan, sometime after our Old Okinawan texts (to 1609 CE), and presumably sometime during Middle Okinawan (from 1609 to 1879 CE), though it isn’t clear to me at this point when this would have occurred—especially owing to the fact that I don’t have access to the Ryūka Zenshū, the major collection of Middle and Early Modern Okinawan poetry, but if memory serves me correctly, this sort of construction is not attested in the Ryūka.
This reanalysis took the derived, morphologically complex construction of the infinitive form of the main verb plus the auxiliary verb or- ‘to exist’—but in this case the progressive auxiliary, and reanalyzed it as the basic, morphologically simplex imperfective form of the verb, an analysis first proposed by Hattori Shirō (Vovin 2009: 611).
So for instance, the verb ‘to approach’ is given as Old Okinawan <よる> yor-u |approach-ADN| in the Omoro Sōshi (OS I: 13), while it is Modern Shuri Okinawan <ゆゆる> yuyu-ru |approach.IPFV-ADN| (from pre-Modern Okinawan *yor-i or-u |approach-INF PROG-ADN|).
The second morphological change, which post-dated this first change, is another reanalysis. This time, an extension of the use of the adnominal -ru with both consonant and vowel stem verbs, rather than just vowel stem verbs. Historically, consonant stem verbs would use the allomorph -u.
This can be seen in the example above, but for a further example, compare Old Okinawan <てる> ter-u |shine-ADN| with Modern Shuri Okinawan <てぃゆる> tiyu-ru |shine.IPFV-ADN|.
So in summary: two innovations which can be used to divide Modern from pre-Modern (Middle?) Okinawan are the reanalysis of the morphologically complex progressive construction as the morphologically simplex imperfective form of the verb, as well as the expansion of the adnominal -ru to all verb classes.
Update (23 December 2014)
I found the verb form <待る> MAT-Uru|wait-ADN| (the capitals indicate my inference of a semantographic Chinese character) in the Ryūka Gimon Roku (Gyokuzan 1900: 2b). Compare Western Old Japanese mat-u |wait-ADN|.
Reference
Gyokuzan. 1900. Ryūka Gimon Roku ‘A List of Questions [About] Ryūka.’ University f the Ryūkyūs manuscript. URI: http://ir.lib.u-ryukyu.ac.jp/handle/123456789/10289
Vovin, Alexander. 2009. A Descriptive and Comparative Grammar of Western Old Japanese. Part Two: Adjectives, Verbs, Adverbs, Conjunctions, Particles, and Postpositions. Folkestone: Global Oriental.