conversion of pc supply to high power rating supply

if any body have som idea, or whether it is possible to convert pc supply to high power supply of rating 12v,20 A

Yes,

It involves removing the transformer and rewinding it, deleting all the irrelevant outputs and calculating new resistors for the voltage monitoring circuitry...

Thats for starters....

PC supply upgradation

what about MOSFETs ? Whether already existing MOSFETs will be OK with new required rating or , we have to change them also with new ones.if so,any sugesstions plz for 400v, 20 A

If you are only changing to 12V, 20A, it is still only 240W. If that is still within the original total rating of the supply then the primary circuit should not need to be re-engineered.

If you're expecting to increase the total power out, then it may not be possible in the same package, even with advanced skills.