[dsmid]

Originally Posted by ixtab

Nope. The upgrade button will remain grayed out if you are on 5.4.2.1 and try to install the "original" 5.4.2 .bin

I've done my homework, tried to force the update and got this:

   140224:104301 otaupd[2058]: I ota_lipc:startUpdateThread:battLevel=69:startUpdateScript
   140224:104301 otaupd[2058]: E ota_monitor:GetApplicableUpdateFailed:reason=noUpdatesAvailable:Trying to install null update_file object.
   140224:104301 otaupd[2058]: I ota_check:validateOTAHeader:status=starting:Starting OTA Check
   140224:104301 otaupd[2058]: E ota_check:validateOTAHeaderFailed:reason=updateSoftwareIsTooOld:
   140224:104301 otaupd[2058]: W ota_monitor:rescanOTAMonitorWarn:reason=validateOTAHeaderFailed:
   140224:104301 otaupd[2058]: I ota_monitor:getPatchRev:patchRev=0:
   140224:104301 otaupd[2058]: I url_builder:HandlerExists::Adding/Updating property to handler
   140224:104301 otaupd[2058]: E ota_monitor:GetApplicableUpdateFailed:reason=noUpdatesAvailable:Trying to install null update_file object.
   140224:104301 otaupd[2058]: W ota_monitor:InstallSystemUpdateRetryScanWarning:retryCount=1:Found invalid update, rescanning filesystem.
   140224:104301 otaupd[2058]: I ota_monitor:getPatchRev:patchRev=0:
   140224:104301 otaupd[2058]: I url_builder:HandlerExists::Adding/Updating property to handler
   140224:104301 otaupd[2058]: E ota_monitor:InstallSystemUpdateFailed:reason=invalidUpdateFound:Trying to install null update_file object.
   140224:104301 otaupd[2058]: E ota_lipc:StartUpdateFailed:reason=installUpdateFailed:Unable to install the system update.

The most interesting part is this: validateOTAHeaderFailed:reason=updateSoftwareIsTooOld
It seems to me Lab126 is trying to ban downgrading... I think it's quite rude.
Because if they come with an anti-jailbreak measure in the next firmware release, we're screwed.
The firmware version must be newly stored somewhere in the header, but I don't think it can be modified easily, if ever (because of the bundle signature).

Spoiler:

Fortunately, I was lucky to find a way how the firmware can still be downgraded in spite of this check, but I won't speak about this method in public. PM me if you are interested.