all the above replies are correct. I'd just add to that it can be relatively simple thing to do, on occasion! no one should buy a folder with the idea of swapping out the lens with another unless they have that experience of knowing what fits and what doesnt, 99%of the time it will be money down the drain...
in addition to taking in actual focal lengths as mentioned by GW, there is the shutter specifications to consider, diameter (inner), thickness and thread , their not all the same, even on all, say 6x6 folders
obviously swapping lenses between the same model camera is the easiest option (not always, e.g even agfa isolettes can be difficult, some have 75mm lens other models of different years have 85mm, the lens standard is in a different position and doesnt allow swapping easy at all!), then between cameras of same design but different countries, e.g russian or Japanese copies of german cameras, then between cameras of the same make and similar build is usually easiest. sometimes cameras of different makes (e.g welta and balda and many others, some generic/distributor brands) swap quite easy but you have to know or get lucky with the lens and shutter combos
also the bezel on the front of the lens (that might have a scale on it- for scale focus lenses) doesnt always want to fit into a different shutter (some shutters have built up front panels) or if it does it might hinder the closing of the camera (some bezels can be quite large).
in some instances you find it easier to swap the lens and shutter together (assuming its the right size shutter-not all 6x6 or 6x9 use the same size shutter comparative to their size) but then you may well have a problem with shutter lever being the same or in the same place as the original, even if the shutter (and its markings) are exactly the same model that might be the single difference, you would then have to dismantle the shutter and replace the shutter actuation lever (easier said than done).
some lenses and or shutter combos can be swapped with TLR cameras of similar years, even some TLR's lenses and or shutters from later years will fit folders with good results (some have wider coverage which equates to sharper images, less fall off to the edges) but the same caveats apply. swapping lenses can give you an interesting combination, Heliar into a Welta Weltur is an expensive option but is easy with a good result, quite a few others are good too, but it is a s l i p p e r y slope to want to swap lenses ..
i dont avocate it, better to keep it original, buy another identical would work out cheaper in most instances...it might sound like i do it all the time but thats just because i mucked around with heaps of them, you get to know whats fits what and sometimes i like to compare lenses so i swap them around and then put them back, usually its easier to put them into a folding plate camera to test comparisons
cheers
chippy
