i am new to the rooting world.and i have just installed a custom JB rom on my sony xperia U.now i want to change my rom.but i am confused weather to again flash the boot.img file or not.my recovery is clockworkmod.plz help
It depends on the ROM. Usually on the Rom thread there will be the insturctions whether to flash boot.img or just flash directly.
Normally you flash the boot.img with flashtool and then do a full wipe and flash rom through cwm.