PHP application migration windows server

Good afternoon, guys. I'm having an application on a server with encoding in ISO-8859-1 and I'm migrating to another server with window. The application is working except for errors with special characters. I gave a searched and tried to convert all the files to UTF-8 but I could not and there are many files, type 10mil. When I do one to test it works, but my problem is this, do it for everyone. Anyone who can help with the conversion or with some configuration that solves the problem even without conversion I would be very grateful.