I can say that there won't be 8x23m services from Edinburgh to Fife associated with this batch of units. It would involve lengthening platforms across the route, to an enormously expensive degree, some of which would be very, very difficult to extend; North Queensferry springs to mind.
Currently, services can run with any combination of between two and six carriages, however the fleet strength means that there's a tendency towards the lower end of that on many services. The new fleet will consist of longer units, however this will still max out at around the 6x23m standard. This will help soak up capacity on some shorter services, and standardise fleet length somewhat.
I may need to check, but I am almost certain that every platform in Fife and beyond can platform 6x23m, with the possible exception of Springfield. Certainly all stations to Tweedbank, and basically everything in the Central Belt is long enough.
From memory, the only stations served by anything longer than 6x23m are, and will continue to be, Inverkeithing, Kirkcaldy and Leuchars. I can't remember their operational lengths, and although the cost of lengthening to platform a full ten car 80X combination would fall on Transport Scotland, they will be unwilling to do something for the purposes of four LNER services in each direction per day.
So the answer to your question: "where would platforms need to be extended," should be zero, however the odd platform might need a little extension.
For the answer to "what logistically is involved in such," then unfortunately you'll need to wait on someone who's worked with the civils departments; I'm not qualified to build them, just to point at where they need to be built.
