Bricklink Studio - Replace part with custom part?

[Ryno917]

Hey all, probably something I'm just missing but I can't find an option for this directly.

I've got a MOC all build, instructions sorted out and ready to go. I've finished most of the custom decals for it and started applying them in Studio as new custom parts. Going back to the MOC model, using the "replace part" option I can't find any custom parts in the list. Does this feature not work with custom parts, or is there a setting/option/button somewhere that I'm just missing? Would rather not go through and manually change every single decorated part and move it to the right step.

 

Thanks!