Changes at the elbow are easier to accommodate as the arm naturally narrows. As a result, there's less to reshape and it's relatively easy to alter.
If you've got a separate lower arm, I recommend trimming above the elbow on the Kairic Acolyte to get a flat surface, then gradually trimming away the upper part of the spare arm, dry-fitting repeatedly until you get the position and angle as you want it. If it's plastic, poly cement will go a long way to hiding the join, and you can 'pour in' plastic filings to help fill any cavities.
For the shoulder, I recommend cutting into the torso itself, then replacing the arm completely.
There're two examples in my image below: