Let's Encrypt の RSA 4096 ビットを ECC 256 ビットに置き換える方法

I’m ambivalent on documenting it. The benefits of wholesale replacing an RSA certificate with an ECC one are so niche, and the potential downsides so great, that we’d probably end up with more “I did this thing without considering the consequences AND IT’S ALL YOUR FAULT” topics than “I have a legitimate use case for an ECC cert but can’t figure out how to modify the template to make it happen”.