Can I change the page count after exporting?

Yes. Update the page count in the Cover Maker and export again — re-exports don't cost anything.

The important part: page count drives spine width, so if your interior's length changed, your old cover has the wrong spine and must be re-exported before you upload it. This is the single most common reason a cover that "was fine" gets rejected on a later upload.

The reliable workflow is to finalize your interior first — the Interior Formatter reports the exact page count and writes it back to your cover — then export the cover last, so the spine matches. See Does the new page count update my cover?