I think Andrew missed the fact that if you just turn on multi-currency all those vendors and customers that you have called USD or GBP or W.H.Y. (typically by way of a Control Account Set) remain as single currency (DOS) customers and vendors when now you want them to be multi-currency (USD or GBP or W.H.Y.) customers and vendors.

To do this properly takes time as there is no easy fix. To only option open to the end user is to create new multi-currency customers and vendors but this means abandoning the customer and/or vendor history.