Just want to make one correction. Serbs have been using Cyrillic since middle ages. It was reformed in the first half of 19th century. Latin script was introduced into Serbian language also in the 19th century, mostly because of unity with Croats, but it really didn't take off until second half of 20th century.
Given that Serbia is more or less independent since middle ages in TTL, I would say it would still keep Cyrillic and wouldn't adopt Latin at all. My guess is the middle ages Cyrillic would be reformed earlier than in OTL, maybe even in 17th century. Dacians would keep Cyrillic alphabet as well, and will not change it to Latin as Romania did in OTL.
It is even somewhat possible to see Croats adopting Cyrillic as their main alphabet in case they are separated from Hungarians and they are in Rhomanian/Serbian/(maybe even early Yugoslavian) sphere. It would really depend on if Serbia can expend north and /or west. My guess is maybe it would be tried in some time, especially it is only available path for expansion, east and south is Rhomania so no way to expend there. Will Serbia try something like that? Not sure if Rhomania would allow this. Only if Hungary tries to break off Rhomanian influence, which is in my opinion plausible, but not very likely.
And speaking of South Slavs, what is going on with Slovenians?