Ideally, the systems are well designed and integrated, there will be minimal impact on business processes, and most transactional activity will be automated. Therefore, changes to human involvement in the business processes should be minimal and only a small amount of update comunication/change mgt/training should be required. One person working centrally can configure the new VAT arrangements centrally and set up changes to roll out in a controlled manner.
In reality, hardly any companies have such idealised systems. Different divisions are running completely different systems, there is some p.o.s. legacy system underpinning the whole thing that will require numerous manual changes before it throws a hissy fit about rate changes, and there is already a tonne of rework required to reconfigure systems and reconcile accounts for the last tax year. A whole team of people have to go round, reconfigure the ERP systems with the new rates, test the new setup, chat up the b.o.f.h. who knows how the legacy system works, fix the legacy integrations broken during the reconfiguration, manage the change, design the transition schedule, train the accounts staff on the quirks and nuances of the latest calculations so that they can reconcile year-to-year figures.