Okay, here's a little more strangeness:
I found a Dim A, d, tb As String (where the "a" had changed to "A") in a sub.
When I change "A" to "a", I can see it change throughout the Sub. I change it back to "A" and it changes throughout the sub again -- immediately, as soon as I type it. The same thing happens when changing "d" to/from "D".
When I changed "tb" to "TB", it changed in the sub, but when I changed it from "TB" to "Tb", not only did it not change, but "Tb" turned red (the color I use for UndeclaredIdentifier) in the Dim line and "TB" turned red in the sub. When I changed "Tb" to "tb", the code throughout the sub also changed and it was no longer red. Then when I went from "tb" to "Tb", this time it did NOT turn red. Then I went from "Tb" to "TB" - not red; back to "Tb" - not red. I could no longer make it turn red again.
I changed it back to "Dim a, d, tb As String" and then changed "d" to "D". This made "tb" turn red on the Dim line and throughout the Sub.
Even though all the instances of a single-letter variable in the entire code changed at the same time, changing the Dim in a sub back to lowercase only changes that sub.