I just got a used PSP with CFW 3.90 ME-2 and updated it to OFW 6.61 by placing the EBOOT.PBP into ms0:/PSP/GAME/UPDATE and executing it.
As I have an issue connecting to my PS3 (OFW 4.82) I wonder if the flashing of OFW 6.61 really removed all changes of that previous CFW.