I thought the IS250 had a hard drive? If so I can't help, but if it still runs on disc then you need to purchase the later discs from 2009 on or borrow one to load, as once that's done the software change is permanent to allow changes on the move, even when you put your old disc back in.
Thanks Tango, the cars still new and I'd not tried the sat nav out properly, just assumed it was like my 4 year old IS220d