Is it common for carriers to have separate certification for analog versus glass cockpits?
Pilot type ratings are usually same for both versions (analog and digital cockpits) of the same aircraft as far as regulations are concerned. For transition to glass cockpit, you may have to take a Electronic Flight Instrumentation Course (or differences training course).

In some cases of transition (for example, EASA  ATR 42/72 (glass cockpit, non 
ECL) to any other ATR 42/72 'classic'), a full type rating training course is required; however, the type rating is still the same.

For example, see the FAA and EASA type rating lists.