Can I create an autocommand to be launched when I execute :update?

I would like to do some code reformatting whenever :update command is executed, is this possible?

I can't find any autocommand hook for updating, only for :w command.

:update basically is execute :write if buffer is modified, else do nothing. The same BufWritePre / BufWrite events apply to it (when the former case is true). That should be perfectly suitable for your reformatting trigger.

To only trigger on :update, but not on :write would require re-writing the :update command itself. For interactive use, that could be done via the cmdalias.vim plugin, which allows redefinition of lowercase built-in commands.