This page describes how to revert to RedBoot bootloader if you have flashed Apex to flash.
Beware that if you make a mistake you could corrupt your bootloader, requiring JTAG hardware to fix it.
If you have APEX installed, then you probably already have a serial console.
Use an original Linksys image file or an official SlugOSBE (aka OpenSlug) image file for flashing.
Send over the flash image from your terminal using xmodem protocol. This will take around 17 minutes to transfer over the 8M image. Please be sure that at the end of the process APEX tells you that 8388608 bytes were transfered.
Edit the image by adding the unit's MAC address (printed on the bottom) at offset 0x03ffb0 of the image. It should occupy 6 bytes. An alternative is 'copy nor:0x03ffb0+0x06 mem:0x103ffb0'. It is very important that the MAC address in the image is correct.
If you wish to copy the existing SysConf info (mtdblock1) into the image, the command would be 'copy nor:0x040000+0x20000 mem:0x1040000' This assumes you loaded the image starting at 0x1000000 as described above.
Now, it should have RedBoot running and your slug booting like before. If not, take your JTAG interface and reflash your slug properly !
Based on work by sdm485 and mathieu_xwave.