The best course of action would be to create a new record for that client, with the new currency. Any ongoing jobs for that client can be cancelled and re-entered on the new record with their totals recalculated accordingly.

The best way to do this is as follows:
1. Clone the client (select the client in the list and click the "more..." button (the one with three horizontal lines) then click "Clone"), 
2. Change the clone's currency and adjust their prices accordingly.
3. Mark the uninvoiced jobs on the old client account as "cancelled".
4. Manually re-create the jobs on the new account.

Simply changing the currency on the existing record is inadvisable, since it will also change the currency of all past jobs for that client, while keeping their totals the same (with no conversion).