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Hi Tymen,

Great questions, to answer them:

1. Yes, you should try to change your backlinks to match your HTTPS adress but don't sweat it if you can't change all backlinks.

2. Try to avoid 301 redirecting internal links. There are many great guides to do HTTPS migrations in Wordpress and doing complete rewrites from HTTP to HTTPS.

3. It might affect your rankings short-term, but that's not necessarily the case. We've enables HTTPS on multiple websites without any noticeable drop. Be sure to check out these great guides to help you out:

https://moz.com/ugc/increase-conversions-by-fixing-https-errors
https://moz.com/blog/enabling-https-without-sacrificing-web-performance
https://moz.com/blog/seo-tips-https-ssl

Hope this helps!