Supplementary question - how does this affect type ratings? Does the -300 rating specify EFIS or non-EFIS? What about a -400 rating does that cover things like the 700/800 and BBJ2?
It ‘s hard to say because every country has its own regulations concerning airplane variants, same type rating, crew qualification, cross-crew, etc. However pilots must have a minimal differences training prior to flying both variants (it depends on what kind of previous pilot experience, so some levels of differences training are required, for example, from just a day class until hands-on training on simulator and/or real flight) and have to be current flying certain number of hours during a period of time, usually no more than 90 days. In my country pilots are not authorized to fly 737 EFIS and non-EFIS airplanes using the same type rating, but they can fly all 737 EFIS family using a single license. However they must be scheduled to fly regularly both series - 737 Classic (-300 and -500) and NGs (-700 and –800).
G'day.