I actually did a couple of conversions on the premium plastic
BattleMaster, maybe you can use these as inspiration.
Here's a BLR-5M:
The limbs are actually super-duper easy to work with. Here's what I did:
For each knee you make two cuts with a sharp knife, one up underneath the kneecap (red line), and one back along the circular servo (blue line). Then you can shave down the corner on the circular servo, and reposition the lower leg at any angle you choose; I'm particular about pinning, so I added a metal pin where the purple line goes between each part of the leg. Superglue, and boom, done.
HOWEVER! This is gonna look funky if you don't reposition the thigh-hip joint (red circle with arrow). What I did was trim off the top of the leg (thigh), so that the leg could rotate just a bit away from its stock position. It might take a little fidgeting to make this work; the peg and hole for these two parts is pretty well fit to begin with, and you may need to take some material off the peg up inside the hip cowl. Doubly-so if you're counter-rotating the leg away from the direction from how the pin and hole mate; this makes more sense when you have the two parts in hand. Just make sure to dry fit things a few times before you glue them.
Other than that, glue and move on!
Repositioning the arms is really trivial; the shoulder socket allows perfect rotation, and then you can either set it and forget it, or for a more dynamic pose you can bend the arm at the elbow. For this build, I cut the arm apart at the elbow joint (red line), added a bent pin (purple line) between the parts, and then glued and gap-filled with epoxy putty. (Worktroll's method with modeling clay would work just as fine here).
As for the rest of this model, I trimmed the shoulder armor for a smooth look, cut the SRM launcher, and added a MWDA gun bit to the arm for the Light Gauss (or ERLL, which barrel is which is not actually important).
Second up, here's a BLR-1S
For this one, I did the same thing to the legs and arms as the -5M. I cut the barrel off the right arm and drilled two holes for the SRM-2, added a bit from a MWDA fig for the SRM-2 in the Left Torso, and then added the BIG HONKING LAUNCHER on the left arm by using a tube of styrene and some epoxy putty. Arms up and in firing positions, on the run!
The most important thing when making these alterations is USE A CLEAN, SHARP BLADE. The plastic cuts with very little resistance if you're using a good sharp edge, and then you don't have to force it and potentially make a blood sacrifice in the process.
Here's some more pictures, if you're interested in seeing a few more angles:
https://imgur.com/gallery/qbLsXXlGood luck, happy converting!