Having nothing specifically to do with Obis yet more to do with downloading...
Here is one reason certain devices reject the ability to downgrade (revert) to older versions of firmware.
Many devices use c0de in a specific part of memory to download or bootstrap. It controls the actual reception of a download file and knows where to put the new code. It is not unusual for a company to have to change the downloader or location that the new code must reside. Perhaps they store all variables in the top of memory and then code follows.
Perhaps because of the length of code, the downloader now adds checksums and the earlier version does not have them. Not all downloaders are backwards-compatible with older software because then the downloader keeps getting larger and larger to support older code formats.
The downloader is not changed on every software update, so with an updated downloaded and a mismatched version of software, the downloader may corrupt a units memory and there may be no way then to update the downloaded.
This happened on a mainframe to me once. The disk drive thought it was a lot larger than it physical was and the system told to disk to go seek out in "la la land" (a technical term) and the physical disk was "toast" (another hardware technical term).
This may not be the case with the Obis and the version that you are loading, but you might keep it in mind if all of a sudden you get odd behavior from the unit (garbage data, entries that won't update, etc).