Well, its actually very simple, open the SFO in a hex editor and edit the FW version. The problem however lies in sending the SFO back to the PS3, it is not that easy.
Furthermore in many newer games the executables (ie the ELF/SELF) have the compiled with SDK Version embedded in them, and they fail if the version of the PS3 is not equal to or higher.
Also, check out
http://brdgen.ps3news.com/ , near the bottom there is an SFO editor for the PS3 and PSP, it should do the job for you if you are not handy with a hex editor.