Amgen officials notified the city of the change late Tuesday afternoon.
The racers will start their leg near Morro Rock, heading north along Highway 1 to Monterey.
About 1,500 to 2,000 spectators are expected to attend the event.
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The city has announced that a beach party, open to the public, will be held at the Coleman Park parking lot from 8 to 11 a.m.
Local businesses and nonprofit organizations will be on hand with barbecue, ice cream, fudge, kettle corn, coffee and more, said Sam Taylor, Morro Bay’s deputy city manager.
Some of the city’s main roads will be closed for about 30 minutes during the time the riders are making their way from the rock to Yerba Buena Street and out to Highway 1.
Those wishing to view the race start at Morro Rock are encouraged to park their vehicles away from the route and ride their bicycles or walk to the event area, as vehicle parking will be limited.