Good question, pinging Jamestown.
“And what about the currency in those countries that use her image?”
I would assume that new currency will picture the King, and the old currency will remain in circulation. There may be commemorative issues featuring her into the future.
She is currently only on the $20 banknote in Canada. Used to be on them all once. Also on all coins still though. The money is still good. Older coins with her father on them were as well.