If the file says incompatible on the loading screen, then that's intentional. I checked some changelogs and there were couple of big changes between version 24 and 28. The unlocks and settings should work fine, so you can overwrite the old version with the new. Old saves won't work, but unlocks will stay.
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Not really. But if you downloaded new version and overwriten the old without realising that your old saves would be incompatible, then you can download one of the older version and finish your old save on that older version. After that, you can download the new version again and overwrite you old version. Your old saves won't work, but you already finished the game on the older version.