These cameras had magazines that could be exchanged in the middle of a roll (and that was one reason NASA chose them).
Here's a photo of John Young exchanging a magazine during an EVA on Apollo 16:
So, no need to wait until they were back in the LEM.
This long PDF has more detail on the film change process. A standard Hasselblad magazine has a darkslide: a metal sheet that forms a light-tight seal over the magazine. This means a photographer can swap magazines in the middle of a roll without exposing any film to light. Insert the magazine, then pull out the darkslide and you're ready to go.
The cameras NASA used were modified: they included a reseau plate. This was a glass plate with cross markings to help gauge distances and sizes from a photograph. The presence of this plate meant that the astronaut had to remove the darkslide before attaching the magazine to the camera. So some film was exposed during a camera swap, and astronauts routinely took some blank shots to get any exposed film out of the way (and test the film transport).