Lets say for example I have:

class foo1 {
String a;
String b;
}

and I have literally hundreds of different objects of class foo1
serialised to different XML files. Now I need to modify class foo1 to
include a new attribute so its class definition is now:

class foo1 {
String a;
String b;
String c;
}

What tools/approaches can I use to modify all of my XML files to
include the new attribute 'c' with a certain value say 'newattrvalue'.
I need this tool to be fairly powerful but easy to use because we are
in a commercial environment and can add/remove complex attributes from
existing classes that are not of simple types String, int, boolean, etc.

Thanks,
Stephen