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Go ahead and change it as of when your 2018 sales are done. It has no effect on prior transactions.
John is correct, but I prefer to err on the side of caution
create a new sales tax item and make the old one inactive
if you made a sale in Dec, and have to refund, process a return, etc etc in Jan, if you do not have the old sales tax rate item available, the new sales tax rate will be used