In addition to hardware differences, there could be sql data base structural differences between versions.
Those are not changed during the image change, they live in a different partition.
When go up in version number, the new image will include an update script to move the user's data to the new structure.
There isn't any factory scripting to go from new structure to old structure.
Of course, those differences are only hit when old firmware tries to reference data in a new structure table.
Which is where the device's usage history can come into play.
If their is never a reference to a new structured table (because it is empty) by the old firmware - no hang.
Those tables in the user's persistent storage do not 'just happen'.
There are reference copies in the main filesystem that are copied into the user's persistent storage during the "Reset (to factory defaults)".
Which is why the old, but still the most recent, jail break directions told people to "Reset" the device before installing the factory firmware image.
I.E: Brand new, empty, database tables.
If empty => no references => no chance of hang from this problem
= = = =
See, the crazy old man isn't all that crazy.
