Renaming the extension doesn't change the file contents
and Amazon isn't that buggy that they won't notice. They'd think it's corrupted at least.
...
Convert to dual-MOBI
Preferences ==> Conversion ==> Output Options ==> MOBI Output (tab)
MOBI file type -- set to "both" in the dropdown.
You can also use Amazon's official tool "kindlegen" which always outputs these.
This creates a file with both KF8 (AZW3) and a mobi7 fallback.
Amazon's Personal Document Service accepts these dual-MOBIs and will deliver an AZW3 to newer devices.
Sending a plain old AZW3 will not work.