If you add the new form to all your existing projects, and then remove the old form or somehow delete it, then you only lose all the answers that people gave to old fields that were on the old form (that you've chosen not to put on your new form).
Users seem to disagree on whether or not this is crucial (history-wise). On one hand, they are no longer interested in capturing the information. On the other hand, they used to be interested.
We usually keep the old form information around for some length of time (say, a couple of months) but make the visibility admin-only. This way they figure out real quick what they're missing and it's still retrievable. Else we give them the option to download the goodies before we remove the old form.
Good luck!