Consequence of changing running workflow task

In short, what would happen if I change one form of the workflow which has over 400 running instances? What are the potentials pitfalls of doing so?

I'd like to add several more fields on initialization form (of the workflow's task which is just an intermediate step in the workflow) which would, ultimately, map to the list item on which workflow was originally started.

The message I got when I tried to edit the form.