is it possible to apply changes on already existing step or igs file?
do you mean with "Changes" modify the file?
changes means editing features....like thickness, hole size or fillet..can we edit in step or igs file itself?....
can we get tree diagram of each n every feature rather than just only partbody??
For thickness you can change it with "Thickness" command as positive or negative. But for diameter and radius u cant. For these you can follow that step. Firstly for hole and cylindirical pocket. Create a sketch on the surface and project the curves and make them construction. then create point/points on the center of circles (if they are patternable you dont need do this for all) and exit sketch. Run the "Romeve Face" command and delete the surfaces which you want. And recreate them with new value.
Secondly for fillets. Run the "Romeve Face" command and delete the fillets. And refillet with new value.
In addition you can resize hole diameter with Thickness command if you select circular face.