There's supposedly some way of increasing the number of Sisters of Battle by using Sisters of Silence, but I haven't found the details. It sounds like one kit has more bodies than weapons, and vice versa?
Not sure, but the sisters of silence kit comes with 3 full sets of weapons, 2h swords, flamers, and bolters and the arms for them are probably close enough to the sisters of battle arms. The bodies look completely different though.
but if you're going to 3D print then why bother with plastic in the first place? Others are buying certain sets that are out of print (buying the 9th starter kit instead of necron warriors).
Filament 3d printers have made some pretty big leaps in improvement over the years. The bambu labs stuff is basically plug it in and go and get good quality prints without tuning, needing to spend the first week printing braces and shit, and so on like the old ender 3 that seem like a dime a dozen these days. But filament prints look like shit for anything beyond terrain. For resin, I actually watched this video the other day, and it covers the issues.
Basically, you still have to tune the shit. You're effectively handling toxic waste even if you aren't concerned about the disposal. Some resins are far more fragile than others(over exposure also impacts this), you need a setup to wash all of this shit, and cure the shit, and the interface on the printer needs to not be dumb. You need a ventilated room that's also warm, if you have a pet they can't be near this shit. Cleaning up what basically looks like pimples from the supports at times is more of a pain in the ass than dealing with mold lines. Need to use superglue instead of plastic cement. Slow as fuck even on the fastest printers if you go for the thinnest layer lines. Layer lines still visible(and made worse with some wash or dry brush techniques) that can require sanding and filling to fix. Sometimes having to experiment with the orientation of the parts you're printing(which adds up to using even more time, the space you're doing this in occupying more time, and so on), film failures resulting in drowning the printer itself in resin and possibly leaking into the printer itself and ruining it(the film in the resin vat is a consumable).
If you know a guy who is fully setup and good to go, and will print you stuff if you can find the models and cover his material costs and maybe a pack of beer or whatever, great. But resin printing is still its own extensive hobby and some people just want to at least minimize how much they have to deal with it.
I bought second hand bits a few years ago. Things like legs and torsos, you used to get a few more arms, Assault Terminators came with double the arms for example.
I don't know if current kits still do that, and tournaments definately won't like it, but for friends, why waste it?
Tournaments only care that your stuff is painted(so it doesn't look like shit), and has the right weapons(if the data sheet says it's supposed to have a big axe, then it shouldn't have a big gun instead), and is the right size for what the model is supposed to be(so you're not modeling for advantage). Definitely not going to care about just arm swaps, and if anything consider that most tournaments having converted/sculpted/modeled stuff done well is an advantage for when it comes to a best painted army award. This is also why tournament organizers ask people to email them pictures of things in advance if they really aren't sure because you've drastically changed a pose or something.