Can I remove 35mm film mid-roll?

I have a Diana+ camera but barely use it because 120 mm film is expensive and so are the developing fees. Lately, I've been getting the itch to shoot again but have decided to shoot with 35mm film instead since it's cheaper. Not a big fan of sprocket holes but I suppose it's good to start small. The only problem is I don't have 35mm film readily available except for in one of my old film cameras (Olympus TRIP AF 30). It's already advanced 1 exposure. Can I remove it in a dark room (not darkroom) and place it into my Diana without exposing all the film?

you have to remove it and put it in the camera in total darkness. The film is so senitive that even a red light will damage it. If you have a bathroom without windows or something like that you can just change it in that or in a changing bag. Just make sure that there is NO light at all.

You will need to do the swap in total darkness. A changing bag is ideal for this, but a dark closet at night works well, too, as long as you can't see anything at all after your eyes have adjusted to the darkness (about 3 minutes). Once the roll of film is in the Diana, advance past the first few frames, tripping the shutter with lens cap on, if necessary. The rest of the roll, of course, can be processed as normal, and you'll still have the one frame from the other camera intact.

Old film shouldn't be an issue, despite the dire warnings on film packages. I have used unrefrigerated film that was 6+ years past its "expiration date" with no discernible loss of quality. And you're using it in a Diana, anyway. If anything, you'd be adding character to a Lomographic image.

I hope this helps! Good luck!
Darin

You can use it as is. The only thing you'll lose is the one exposed frame. Alternately, you can do the switch is a darkroom and lose nothing. The other way is a bit more complicated: rewind the film into the canister; when it's out of the camera, turn the spool clockwise until you're pretty sure the end of the film is at the opening of the cannister; take an old piece of film and lick the end of it, slip it into the opening - the two pieces of film will adhere and you can gently pull out just the leader. You may have to do this several times.
The one problem in all of this is the age of the film in one of your "old" cameras. If it's over a year, or has been in a warm place, forget it.