Can I update a sales tax rate without losing the previous rate? Example: The sales tax rate for Wahoo, Nebraska is 7%. It will increase to 7.5% tomorrow, 1/1/19. Thanks,

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