I have a P320C made in May 2017(voluntary recall never done) and an M18 made in Dec 2020. The older slide will not work with the new FCU. New slide works with both FCUs.

Is there any way to get the old slide to work with the new FCU?

You'd have to send the old slide to Sig to have the recall/update work done, which involves milling a disconnector pocket in the underside of the rear of the slide. Without that added pocket, the old slide can't work with your new FCU.