Routes for large CI3 to CI4 upgrade

This isn't what I was asking. We actually have CI3 and CI4 running together with CI4 in a subdirectory.

What I'm looking to do is have it so when I convert a page to CI4, we route to the CI4 subdirectory. Otherwise, we route to CI3. So links in the CI4 page would still go to CI3 unless there is a page in the CI4 subdirectory. Since we have so many pages and the conversion is estimated to take a year we are hoping to roll out as we go and not wait to convert everything to roll it out.

Any ideas on how to route like that?

Thank you!