I have a site that I maintain (trellis/ bedrock, sage) and I want to switch it to https:// before I do is there anything I need to be aware of in terms of redirecting existing urls/ inbound links? Are there any potential gotchas to be aware of?
Trellis automatically handles 301 redirects from http to https.
It would be up to you to contact external sites to have them update links pointing to your site, but the 301s would handle the redirect.
If you have any external services set up to ping the site, e.g. Google Analytics, Google Webmaster Tools, etc., make sure to change those to HTTPS!
This is a handy guide