You have several options here.
– You could use the save form in document feature, but this would mean that you would have to re-open each existing doc and re-save, so that the form could be embedded in the document. Each doc would get larger, as far as disk space is concerned, with a bunch of redundant data. Personally, this is not the route I would take. IMO, it’s not what the feature was designed for. BTW, the designer help has some decent information on this.
– You could give your new form a new name.
– You could rename your old form. This would also involve updating the “Form” field on the pre-existing docs (a simple agent could do it).
– You could keep one form and use some “display when” logic to display/hide the old/new field information based on something like created date or the pre-existence of the old field information.
– You could use a form formula in your view(s) to decide which form to use (old or new) based on something like created date.
There are many ways to skin this cat. It really all depends on the specifics of your situation.