This is not the reference that I wanted to give, but rather than giving nothing, I'll give you something difficult to use but rewarding if you succeed. In this work is a description of the Ophitic dialect (Spoken in the Of valley of what is now Turkey). The dialect retains the infinitive in its syntax system, and the speakers of some dialects use an "e" sound for η. It is considered the dialect of Modern Greek, which has remained the most similar to Ancient Greek. The journal was written in the late 19th century. As for the subsequent history, following the population exchange between Greece and Turkey in the early 1920's, the Christian population of the region were moved to Greece, while the Muslim Greek speaking population have remained in the Of Valley, speaking their dialect unaffected by Greek media and culture.Stephen Hughes in the - Is this a viable pronunciation system thread wrote:There is a small quantity of standard Modern Greek words, where a Koine eta is preserved as a Modern epsilon - the Modern spelling reflecting the inherited (folk) pronunciation, rather than the (Classical spelling) conventional.
The Article starts on page 120 of the Syngramma periodikon, Volume 18 of the Hellēnikos Philologikos Syllogos (Istanbul, Turkey) (on Google Books). It is written in high-register Katharevousa, which is practically Koine (as opposed to the lower register Katharevousa that older people are sometimes able to speak).
I realise that it is not the best reference work, but on page 136, we can see that in the Of Valley they pronounce θήριο (i.e. θirio) while in Trebizond (in the West of the region of Pontus) they say θέριο (i.e. θerio). On 157, you can see πελό for πηλός in ζησινώ (the final sigma of the masculine nominative singular has been lost in that dialect, but retained in Standard Modern Greek as being basically the dialect of the region around Athens, and because it is close to Koine Greek). There are other examples if you care to wade through. Opposed to that on 136 (again) ήλεο is recorded for ήλιος in both the Of Valley and Trebizond.
The point being, the loss change or not change of η to "i" is not clear-cut and varies across regions and within dialects of Modern Greek. While in the main, η changed to "i", there is no single line of change for the η to become "i". It happened or didn't happen in different dialects in different ways. For η, the picture is a lot more fluid in the Greek speaking world which has evolved from the Koine, than the current situation of having a single standard Modern Greek variety would suggest.
A bit esoteric, but perhaps of some little interest.