I am planning a major overhaul of a well established domain (60 Moz DA). Aside from a platform change, content reduction/consolidation and branding update, the structure of the site will be drastically changed. To me, even with redirects, this in itself is a major SEO strain on the site. I anticipate 6 months recovery time before everything is back to normal.Would it make sense to do the conversion to https at the same time or should I do it in phases? 
Switching to https during a major redesign
seo;301 redirect;https
Do it all at once.A https switch is not going to impact you more or less if you're changing all the content. A new index will be built and crawled for your site - and your old 301 redirected URLs will be merged into the new index entries.In addition to this, because the benefits are long term, the earlier you do it then the sooner you'll benefit. 