Your blend file must be structured exactly as the one you are replacing.
alBody_EP04CostumeLlamaDog has 3 meshgroups:
0000 - almost full outfit
0001 - paws, snout, belly;
0002 - bottom of sleeves.
Each of these meshgroups is affected by specific bones. To replace them without problems, you need to split your mesh the same exact way, assign corresponding cuts, and
clean up the weights, so that each meshgroup is affected by the same bones.
- join all 3 of your meshgroups into one, name it something;
- delete all vertex groups;
- save the blend, close;
- clone alBody_EP04Pajamas, export the blend, open, do not join meshgroups;
- append your mesh from the other blend, transfer weight to it from
the outfit itself ( cut 0000);
- delete the original garment, give yours cut 0000;
- select the other meshgroup and delete only the legs from it because they poke through your mesh;
- save, import back into alBody_EP04Pajamas