Well, here we are 2 years later, and everything is changed. But it’s SO much easier now.
Go to: Gateway → Config → Networking → Web Server
Then: SSL/TLS → View Details button
Then: Upload Trusted CA-signed SSL Certificate (Link at the bottom)
Now you just have to drag and drop your new server, then intermediate, then root file (In that order) into the boxes.
Don’t worry about any kind of a new CSR or anything like that.
Hip-Hip-Hooray for IA!