The IROE (IATA Rate Of Exchange) is normally valid for a month. This can lead to currency losses (and gains) to the airlines if they are being used 'as a bank'. The currency system is in place to facilitate sales across borders but it can be used to 'attempt' to subvert currency export rules and benefit from currencies in a devaluation. So some airlines do restrict reissues that could appear to change the original currency of payment.
For example had your original ticket been paid for in Venezuelan money, changing the ticketing currency to GBP results in the airline having money frozen in Venezuela for months or years, before they can repatriate it at a serious discount.
Since you have no restrictions the original ticket can be refunded, and that amount used as (partial?) payment against a new ticket. It sounds complicated but it really isn't. Any airline or agency should be able to handle that. (Any currency restrictions on the original ticket, if any, may preclude the refund - but that doesn't apply to the euro)