As I understand it... if say you had a replacement set of clocks with the same mileage and knew the SKC for both the you could use VCDS, for mileage changes and/or SKC retrieval you need VAG Tacho also, for one off use these will be a costly option for you to invest in so best to find someone local who can do it for you?
EDIT: As regards changing the actual tacho instrument if both clusters were same ie VDO or motometer? I guess it could be possible but not sure how involved that would be not had them stripped down to have a look myself.