It's possible, but not necessarily trivial. At the very least it requires a two step process.
Disclaimer: I do use several gs1900 devices, but I haven't tried reverting the merged-firmware to OEM myself, so the following is academic and untested! Yes, I did test going back and forth before, several times, but under distinctively different circumstances, as I:
- had serial console access (you really want that, when testing potentially harmful things)
- had full backups of all partitions from the untouched switch in vanilla OEM state
- tested this long time ago, before the merged partitions
- ...and in the worst case, I would have spi-nor flasher, soic-8 clamps and soldering iron at hand, so I could recover 'anything', if push comes to shove
so a quite different starting position.
The below is just an untested high-level overview of what could -probably- be done, serial console access (and flash backups) strongly recommended(!):
- first you need to revert to the dual-partition scheme, namely <= 23.05.x, by reversing the upgrade information for 23.05.x --> 24.10.x
- I would do this by tftpbooting the 23.05.x initramfs image via serial console
- but it would probably also be possible to force-sysupgrade the 23.05.x initramfs image
- boot into 23.05.x, with its split partitioning scheme, make sure that everything works
- now you need to fix/ reflash the (now existing again) second b-partition slot with the OEM firmware
- I would do this by simply rewriting my backups, I took before touching my device
- if you don't have full backups of everything (all mtds), at least take them now!!!
- if you don't have a backup, you must check first how to do this (careful checking the OEM firmware structure/ size of how to write it back (mtd?))
- toggle the boot order and hope for the best...
I strongly recommend to have (and use) a 3.3V(!) usb2serial adapter at hand!
I have never checked the OEM firmware format (if it can be written back by mtd or if changes are required before), because I had my own backups and didn't need to bother about that question. You do want to get a second opinion/ confirmation about the process from someone who has done it this way before. And you want a safety net (serial, spi-nor writer, etc.), don't try to attempt any of this unless you completely understand each step, down to the details and triple check everything - you have one chance and a considerable chance to mess up!
My suggestion would be to first get OpenWrt working as you originally wanted, this is:
- easier
- less risky
- buys you time to investigate the process and get a second opinion