Will having access to Shipping Manager change the existing rates for Yahoo Store merchants?
No, if you are a Yahoo Store merchant, having access to Shipping Manager will not affect your existing shipping rules in any way. You can still edit existing shipping rules as you did prior to this change, including those that utilize the built-in UPS rate table.
However, with the Shipping Manager, when you add a new rule, Yahoo's real-time interface to UPS Rates and Services module will be used in lieu of the built-in rate table for official UPS shipping methods.
Also, when you add a rule, you cannot select an origin zip code; the origin zip code will be pulled from your UPS registration information.