There were several techniques:

• Carry extra film backs (MF) or camera bodies (35mm) with different film loaded
• Push process lower ISO film to sacrifice fine grain in favor of perceived higher ISO
• Rewind the film already in the camera until just the leader is outside the canister, noting what frame you are on, then load a different roll. You can then reload the original roll and expose black frames until you are on the "next" frame
• Use a flash
• Use a tripod for more shots

I've been in many situations where simply walking from a more open area in a city to a denser part where the buildings created shadows made 2 or more stops difference. Carrying multiple bodies or push processing were my technique of choice. Push processing is an all-or nothing thing, so you either push the whole roll or you don't. Push and pull are for carrying one type of film you feel covers the most general case and then adjusting ISO for specific cases outside those boundaries. Carrying a separate body is great because you can switch back and forth quickly. It's not all or nothing, and you don't have to take the time to rewind film, etc.

ISO 50 film, in particular, is dicey because photographers chose it for (among other things) its grain structure. Pushing something like Velvia gives it a different character -- somewhat grainier and even more contrasty. Probably not the effect intended when the film was loaded. Even more so with the slow B&W films like Pan-X.