Hi Alexis,
erm, I think that's correct (strange as it may seem). The revaluation to the last date is what would happen anyway when you run the Adjust Exchange Rates batch job for this date. To make sure I've had a look at Report 86 "Adjust Add. Reporting Currency" - that's the report which is executed when you change the ACY.. It is always adjusting at workdate... so there you go. It also means yo don't really need the exchange rate history.
with best regards
Jens