How can I find all *old* Keynote presentations on my computer?

I have some old Keynote presentations. When I try to open them with the latest Keynote (6.5.2 (2119)), I get the following warning:

This presentation can’t be opened because it’s too old.
To open it, save it with Keynote ’09 first.

I've managed to get Keynote '09 installed, and I can open with that, save, open with the current version, and save again.

It will be a pain to do this with all of my Keynote presentations, but it seems it has to be done if I want to be able to access them in the future. Or I should convert them to PDF so they are in a format with more long-term access options.

How do I find all of the Keynote presentations on my computer that are outdated? Is there an mdfind option that will allow me to find just the old versions?