Update: Keybinding version:

VScode is adding the editor.action.transformToSnakecase in v1.53 so the requested operation can be done easier without having to figure out the neccessary regex to make it work as shown in the previous answer. And because some people might find this question looking for snake case (snake-case) information.

What I show now is NOT a snippet however. You just type your text and then trigger the keybinding. The keybinding itself fires a macro extension command from the multi-command extension. In keybindings.json:

      {
        "key": "alt+3",                        // whatever keybinding you wish
        "command": "extension.multiCommand.execute",
        "args": {
          "sequence": [
            "cursorWordLeftSelect",            // select word you just typed
            "editor.action.transformToSnakecase",
            "editor.action.transformToUppercase",
            // "cursorLineEnd"                   // if you want this
          ]
        },
        "when": "editorTextFocus && !editorHasSelection"
      },

Demo of keybinding version:

Snippet version:

"camelCaseModify": {
    "prefix": "test",       
    "body": [
       
       //  first inefficient try, works for up to three words
       //  "${1} -> ${1/^([a-z]*)([A-Z])([a-z]+)*([A-Z])*([a-z]+)*/${1:/upcase}_$2${3:/upcase}${4:+_}$4${5:/upcase}/g}"

       "${1} -> ${1/([a-z]*)(([A-Z])+([a-z]+))?/${1:/upcase}${2:+_}$3${4:/upcase}/g}"           
    ],
    "description": "underscore separators"
},

This works with any number of camelCase words, from one to infinity...

The ${2:+_} means "if there is a capture group 2 then append an underscore." If there isn't a second word/capture group then groups 3 and 4 will be empty anyway because they are within capture group 2. Capture Group 2 is always the next Word (that starts with one capital and followed by at least one small letter).

for example, using changeNetworkStatus:

Match 1

Full match    0-13    `changeNetwork`
Group 1.      0-6     `change`
Group 2.      6-13    `Network`
Group 3.      6-7     `N`
Group 4.      7-13    `etwork`

Match 2

Full match    13-19    `Status`
Group 1.      13-13     ``
Group 2.      13-19     `Status`
Group 3.      13-14     `S`
Group 4.      14-19     `tatus`

Match 3

Full match    19-19    ``
Group 1.      19-19    ``

Sample Output:

abcd -> ABCD
twoFish -> TWO_FISH
threeFishMore -> THREE_FISH_MORE
fourFishOneMore -> FOUR_FISH_ONE_MORE
fiveFishTwoMoreFish -> FIVE_FISH_TWO_MORE_FISH
sixFishEelsSnakesDogsCatsMiceRatsClocksRocks -> SIX_FISH_EELS_SNAKES_DOGS_CATS_MICE_RATS_CLOCKS_ROCKS

Using regex101.com really helps to visualize what is going on!