Mixed operation in any state of modernization was always possible, except, if I'm not mistaken, around 1964 (RBe 1401-06 and DZt 1901-06).
Very early in S-Bahn-operation, blue and white vehicles turned up. In general, last to be modernized were the BDt. The Dt were always green (but had gotten red fronts).
Electrical and internal modernization was combined with new paint. But the new doors were a single later amendment without other modernizations.
S-Bahn Zürich used four car sets and six car sets for regular lines and sets with two RBe as "Dispozug". These were trainsets ready in Zürich to replace trains that were late or had a defect. So the double RBe sets could be seen on all lines and they were on occasions also sent to replace an IC.
Double RBe sets had been used earlier to replace RABe 12/12.
Shunter's steps were only available on coaches with diaphragms. On the light steel coaches they remained to the end.
Exterior handrails were removed around 1996 within a short time on all SBB, SOB and BLS coaches, independent from their use with or without conductor. (EA 8/1996)