Hey Steve,
I watched Hayhurst do this recently in 5 mins flat. Doesn't matter about whether the tanks are full or empty. Simply go for the central screw, unscrew, remove current knob and replace with 'rubber tactile' knob.
My word this conversation has the margin to go wrong, what?
HTH
Failing that, just bring them with you to Anglesey and we'll get you squared away.