After completing this upgrade, I was not able to initialize the new drive to use as a Time Machine destination from macOS Big Sur (11.6.7). I don't really know why, other than that support for Time Machine over AFP is deprecated and that is what the Time Capsule uses. I was able to initialize the drive by backing up an old El Capitan machine I had to it. After that, Big Sur recognized it.

I would probably not have attempted this upgrade without this guide, so thank you.

A word of warning: I don't know how, but somehow it appears I damaged at least one antenna in the process. My 2.4 GHz network became practically unusable. Top download speed dropped to around 1.5 Mbps. I'm not terribly upset about it, as it is past time to upgrade to a WiFi 6 access point, but it is something to be cautious about.

I replaced the drive with a Western Digital 4TB WD Red Plus which I though would fit without cutting up the rubber, but I was wrong, and cutting the rubber pieces to fit was the hardest part. Another thing I don't like about this drive is it makes a very loud sound when parking the heads, and is not all that quiet in operation.

For me, the metal plate did not "simply lift away". It was stuck to the rubber underneath and I had to pry it off.

I broke 4 of the 8 loops taking the base off carefully (or so I thought). I used iFixit's opening picks to keep the tabs from reseating after I freed them. I found them very helpful.