S3 (i9300/i9305) Is it possible to install CM10 after installing the 4.3 OTA update?
Yes, the reason you can't downgrade official ROMs is not a technical one; it's just a decision Samsung have taken.  

There's no reason at all why you can't go backwards and forwards between versions of Cyanogenmod as long as you have the appropriate recovery. And CM10.1.3 and CM10.2 are currently much more stable than CM11 on the i9300.  

Consult the Cyanogenmod site (http://www.cyanogenmod.org/) or XDA-Developers (http://forum.xda-developers.com/showthread.php?t=2382573) for full details before actually flashing a new recovery.  (Personally I'm staying with CM10.1.3 (http://forum.xda-developers.com/showthread.php?t=2035141) as I found the camera unstable on CM10.2)