as far as flashing custom roms,no. there should be no issue. to my knowledge,JB will work OK on ICS firmware(hboot,radios,etc) and vice versa,so its just a matter of making/restoring a backup.
however,if you mean can you update to official JB,via OTA,then manually downgrade back to official ICS,no,you cannot. the firmware is NOT downgradeable,so you would not be able to run an ICS RUU to get back(unless you are s off.if so,that changes this answer entirely

)
you can OTA to jellybean,and flash a custom ICS rom,while remaining on JB firmware,if you trully do not like JB. IMO JB is much nicer.
hope that answered your question
