I've just begun really making use of tags, so I am unsure if this issue was present in past versions (current version 220.127.116.112).
In the Windows desktop client (Win10), while in the "Tags View" I am attempting to make my existing tags all lower-case. When I attempt to do this, using the tag "Horse" -> "horse" as an example, I am told that this Tag already exists and that I should choose a different Tag name.
In order to change the tag, I must first rename it to something like "horse1" (or whatever), then I am able to rename it to "horse."
Effectively, what should be a 1-step process, becomes a 2-step process (thus wasting 4 seconds instead of 2). I demand my time back!
I'd mention the number of mouse-clicks, but then I realized F2 is the "Rename" hotkey, so that's quite nice actually.