Custom form updates - Affecting only projects moving forward (not changing existing projects)

After using WF for almost a year I now realize our custom forms could use some updating. But I don't want to lose or change any info on existing projects either in progress or completed. I only want to affect new projects/requests.

It would be great if when I make a change to a custom form to be given the option to have the change affect all projects/requests or only those moving forward. This was a great option on other PM tools I've used before.

Thanks!