Not sure what the reason was for switching from Chambery to Grenoble, but based on my experience purely from a passenger viewpoint it makes sense.
Chambery is fine on paper, but does get a bit messy on Saturdays during the ski season especially if the weather is poor when delays can really mount up. Grenoble seems to be less affected by weather issues, and depending on which ski area you're going to is either a bit closer or a bit further away, but no big deal either way.
The advantages of a departure from a local (smaller) airport like Plymouth/Newquay are the key thing in terms of reducing hassle, particularly when travelling with kids in tow. Already booked my flights!