If the currency has changed then it is probably caused by something like importing customer details, there is a warning about this in earlier versions. In later versions you can import a currency flag.
I have some tools that can fix this and put the flag back to the right currency. But would need your data and would have to charge, assuming it is not a hundred thousand transactions you are looking at about an hour to do it so about £50
For just about anything Sage :- switch to renting, pay-as-you-go sage support, sagecover, upgrades, application integration, reports, layouts, analysis or any other help making life with sage easier/less time consuming Contact me