Selected answers from the Dumb SEO Questions Facebook & G+ community.
Ronnie Lawson-Jones: Not at all and maybe a boost if you get the redirects right
Michael Martinez: Never had a problem. Converted over 100 sites. However, if you BREAK the site (inadvertently) that could cause a problem.
Dave Elliott: It`s pretty straight forward. Just sort out redirects, check for any chained redirect chains and update your internal links if you don`t use relative urls.
Doc Sheldon: I`ve done dozens of conversions, and often see just a negligible drop, usually not of great duration - a handful of weeks. In around half, I see no noticeable drop at all.
Gerry White: Depends on how badly it is screwed up - as Doc Sheldon says - having done quite a few now - generally its not noticeable if there is any drop, but after I have typically seen a rise after depending on scale - there are a few resources (including a guide and a HTTPS trello board) here - https://www.takeitoffline.co.uk/https/ - the single `gotcha` is GTM implementation, or wordpress
Antonio Tourino: Thanks so much for all the help, was getting worried about doing it haha