Reissuing a Sales Invoice in a different currency

Hi,

I have a situation as follows:

1. Sales Order raised in base currency (GBP) instead of customer currency (EUR).

2. Sales Order shipped, invoiced and run through Sales Update.

3. Customer has invoice for 100 GBP, but it should be c.120 EUR.

We need to reissue the invoice in EUR, so what would the best method be, please?

Credit Memo for GBP and then a manual invoice for EUR?

Many thanks in advance.

Cheers,

Tom