Root Samsung Epic Galaxy 4G (SPH-D700) on FC09 Gingerbread?
Just about every Samsung device can be flashed with Odin, regardless of what firmware is installed on it at the time. For specific instructions you can refer to this XDA thread, which explains how to flash a rooted FC09 image:


  
  put your device into Download Mode  
  Connect to your computer and make sure ODIN recognizes your device
  Flash the Clockworkmod.tar.md5. Make sure EVERYTHING is unchecked INCLUDING AUTOREBOOT!!!!  
  Once ODIN does its thang, unplug your phone and pull the battery  
  Now reboot into CWM Recovery by pressing and holding the power button, camera, and volume down button until the capacitive buttons at the bottom show  
  Now locate the ROM.zip. It should be called DeodexedFC09.zip  
  Wipe Data/Factory Reset, Wipe Cache Partition, Wipe Dalvik Cache, and if you want Wipe Batt Stats  
  Now reboot, and you should be running Stock Rooted FC09!  
  
  
  ODIN: http://www.mediafire.com/?66csfeo1ce95ysw
  
  CWM TAR: http://forum.xda-developers.com/showthread.php?t=1357655
  
  Stock FC09: http://forum.xda-developers.com/showthread.php?t=1580931


You should be able to get the Epic into download mode by doing the following:


  You enter download mode by shutting device completely off. Then open
  the physical keyboard press and hold the 1 key and then press power
  for a few seconds.


Downgrading, rooting, and then updating seems unnecessarily complicated. The updates probably wouldn't apply on their own once the device was rooted, so you'd need to install them manually anyway. You might as well just install the image you want straight away instead, whether that's a stock one like the above linked image or CyanogenMod.