Nothing yet, no, but this will probably be explored in several upcoming books which will venture into the "Dark Times" era as it's becoming known.
I imagine various things were phased in at different times, and probably there was a long period in between when both kinds were in use simultaneously. I doubt Palpatine got up one day and decreed "Thou shalt all switch over to new gear and vehicles today".
If it isn't broke, don't fix it, so I don't envision all the ROTS tech being suddenly retired, but simply allowed to wear out/get destroyed and then be replaced with the OT-style tech.
For example, a slightly more primitive AT-AT is already in use during the Clone Wars. And the Imperial officer depicted in the ROTS Visual Dictionary is about 90% similar to the classic uniforms, with small differences in the rank insignia and an additional seam across the chest.
A few bits I do know:
In the Palpatine Inaugural edition of HoloNet News in Insider #84, a clone/stormie is depicted in the ROTS style armor (but then, this is immediately after ROTS so that'd be pretty obvious).
In a Jedi Purge story in Tales, we see Vader with a bunch of stormies in the classic OT armor. Of course, this was printed before anyone knew what the ROTS armor would look like, and this may have even predated AOTC (I don't own a copy so I can't check). This is supposedly the last bunch of Padawans in the Republic/Empire (or so our hero believes, at any rate, giving us a possible loophole) so it's likely within a couple of years after ROTS, at most.
Not much to go on, I know.