Im currently still in my first Challenge „California“ at 1000km. Is it possible to change the Challenge to „Everest“ if I want to ride for example Alpe du Zwift to collect the elevation?
Or do i then lose my km in „California“?
You can change it anytime without losing any progress… my suggestion is to leave it on the everest challenge until you get the tron bike.
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… then ride around on the Tron bike for a couple of weeks before getting bored of it. 
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Of all the unlocks you get from the challenges, the Tron bike is the only really useful one so I would focus on that first. It’s a top performing bike in the game, whereas the ones you get from the other challenges are worse than what you could buy. They’re not bad, but it’s easy to do better. (Unless you just like how they look.)
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