Weight change does not have to affect good Opera singing. If one has good vocal technique, one just needs to apply technique to the body at hand. There is a waiting time, to become acquainted with the new global hemisphere, the new sound, the new resonance. The problem is that most singers tend to sing from their heart, instead of well established technique. The difference between the Men and the Boys. Technique does become habit, but if the chamber has changed, one has to wait to understand the new chamber. Exercise plays an important part in singing. Make no doubt, "Singing is an Olympic Sport". We need a well oiled machine.
I, myself, have changed sizes so many times. I have closets of clothes and costumes to prove it. I am so cheap that I would never throw anything out, plus I might get back to that weight someday or maybe tomorrow. Have you ever seen a skinny Opera Singer? Lily Pons does not count.
Miss Voigt get healthy. Exercise.