Hi

There is no reason for any website owner to stay on a old theme when customisations are in place. There are two pretty good programs to find all their customisations by doing a compare old theme original against old theme modified and redo them to the new theme release by a compare old theme modified against new theme original.

They are called winmerge and beyond compare. I use them on a daily base to compare code of themes and plugins and redo changes.

Best regards, Neo