Updating Existing Price Lists - Bulk option

Updating Existing Price Lists - Bulk option

Hi,
I know a lot has changes in the pricelist section and you can now import whole lists from a CSV file.. Great!!
My question regards a rather complex issue.. Updating an existing price list.
We have 50 products and 12 price lists, so if we want to update them, we have to go through 600 products.
To make it harder, we don't intend to change the prices by percentage, or by given price (2$ per item). The changes are individual for each item, some will cost more, some will cost less, no consistency.
We have it all figured out in an excel file (CSV).
What will be the best way to change the prices of all our products on a certain "cut over" date?