Sounds like you need to do it programatically... One option is to copy the correct value to a new property on the existing pages, save and then use the built-in convert pagetype funtion in Admin mode. Another would be iterate downwards and copy page by page to another page branch by code.
Just do a straight conversion, mark the old ones as not visible in edit mode and make a new custom property that combines both of the old ones if its not set by itself.
I would have done something like that I think.
What is the best way to concatenate multiple properties into one when converting a page?
The old page type has multiple properties that we need to combine into one single property in the new target page type. Is this possible?