Overwriting wizard written in old style (wizard.interface)

Old style wizards (wizard.interface) do not support the concept of reusability(inheritance). You will simply need to overwrite the whole wizard. To make sure that the click on wizard calls your new code, you should keep this in mind.
New module : Inside the wizard.xml of your module,you should use the code like this:
id="main module.xmlld" #xmlID of your old module,so it now refers to new wizard definition.

model="Same as old menu"
name="Better to have a new name"
string="Your string"/>

The new name should be linked to the new code of your wizard.