I believe it can be done. It is worth it to get an anual pass and visit several parks thru the year.
It depends on how you purchased the pass. If you bought it online, no. If you purchased it at the guard gate and want to upgrade on the same day you purchased it, yes.
Yes most parks will do this. We purchased a day pass in Zion NP and later that day upgraded to an interagency annual pass. It is worth it if you want to visit a few parks.
No you can't
I don't know, you should try to look it up in the national parks website
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