New additional local currency in a productive system

Dear all,

Has anyone added a new local currency for a company code that has transactional data?

We basically have one Irish company code that was set up with GBP as local currency code because it is at the moment part of a British company (it is not a legal entity itself). But we would now to be able to hold our Balance Sheet and P&L in EUR (which is in any case the document currency for most transactional data), especially looking at a future split of both entities.

I understand that it is possible to add a second local currency (EUR set up as currency type 40 or 50 for example) even though there is transactional data in the system, but what are the implications? Can this only be done at year end or could we use any period end to start using this second currency? Would we necessarily need to convert all historical data to this second currency as well?

Many thanks