Yes they can it is also importsnt in how you bought them. If it was via atom or fandango you woukd need to change it there. If it was an actual purchase at the theater then you woukd go to their customer service.
As long as there's space you can
I've never had an issue swapping tickets out for a different showing. There has been the rare occasion that they gave me a refund in the form of a gift card
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