This article from Threads #177 (February/March 2015) by Linda Lee is a useful technique to know when you’re making alterations.

Altering one part of a pattern often has repercussions elsewhere, especially when working around armscye curves. With one small pattern adjustment, the sleeve may no longer fit into its armscye. An easy way around this is to preserve the shapes by tracing the armscye curves before making adjustments. Once the alteration is made, reapply the traced armscye and true the seams to compensate for the change. Since the curves’ dimensions are the same, the sleeve and armscye still match. I’ll demonstrate four methods for transferring curves to recently altered pattern pieces.