That warning was there to prevent this exact situation. Maybe it wasn't clear enough, however.
The reason the warning exists is because the partitioning layout changed late last year. The changes have been present in snapshot ever since, and a device that hasn't run the newer installer tool isn't capable of running the snapshot images anymore.
You might not have to go all the way back to JTAG. You definitely need the serial console, however.
With luck, you might still have access to u-boot and can replace the broken image via tftp. If you can't even access u-boot and get the dreaded BL2 error, you can follow the procedure here:
and can recover from a very bad flash using the serial console.