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?
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.