Cloudflare account transfer precautions

is there anything to keep in mind

yes there is, because if DNSSEC is active at your registrar (e.g. the “old” Cloudflare account) then your new account will not activate SSL properly, it will be remain stuck and not be issued by Cloudflare.

the same goes for Let’s encrypt certificates and some other cases, in general its a very good idea to temporarily disable DNS Sec for at least 24-48 hours before transferring your domain registrar, your Cloudflare or other DNS management platform, and before making any changes to SSL certificates too.

OpenSSL is not effected by this cuz its self signed and does not require public validation via DNS, but in nearly all other situations its a good idea