There are ways to make this work, but they take some forethought.
As long as you are not deleting fields from the template, the old forms will still function.
Do you have a field in the old form that is a true/false for submitting the form?
If yes, then there are ways to make what you work, this would be the best option.
If no, then your options become limited and those options could cause your form to become convoluted and harder to maintain.