According to standard functionality in IFSAPP10, there is a no-way that can change the Currency Code on the Sales tab after a time/expense entry has been entered. Only the postings entered after the currency changed will be considered for the change. When we enter a posting line it's based on currency type, sales price and customer currency conversion factors. Therefore changing will make so many difficulties in calculation and as well as the process flow.
As you mentioned, only way to dismiss the earlier postings is to use the non-invoiceble functionality.
Just for information.
We are looking into extending this functionality in IFS Cloud (21R2) so that if changing the currency code on the Work Task (not the Work Order) it should modify the all sales lines which are not invoiced with new prices and also the currency code of the sales line will be updated.