The changeover options are discussed in the Pieter Dekker paper referred to above. Modify your current systems Replace with a new multi-currency package that can handle the Regulation 235 rules. Replace with a new Dual base currency package. Replacing or upgrading packages has other implications. The euro needs more than most multi-currency packages provide.
Every business process is affected by the euro.
This must cover setting prices, order processing, delivering, invoicing, and payment systems, with the attendant problems in the transition period of when trading partners go through their own E-day. Ask about the euro migration suite your software vendors should provide. This includes database conversion. That's why business involvement is so important. Easy migration is not, however, likely with spreadsheet applications.