Changing ID Element Tags

How would I go about chaning the ID element tag through Java?

I want to create a function that checks through all the ID tags called mytag and then append to the end of it mytag0 or whatever number comes next. I was trying to disable many form objects in my html document and since they share the same ID It only disables the first ID it encounters so Instead of manually renaming them al, Id rather create a function that would search for the Id name mytag and just change the name with whatever number, Im guessing it will work fine because once it renames the ID tag it will pick up the next ID because it will be next in line with the name and just While Loop the whole thing untill such ID is not found. I do not know how to reassign ID's.