If its about setting up a goal - you’ll set up an event goal and leave the label value blank. Then go to your Goals report and you will be able to see on which page the goal happened.
In both cases, you actually dont need the form ID but are able to clearly see which pages the form submissions are happening.
I understand thats not the answer to your specific question of how to change form ID of a form but I am just suggesting from a user perspective. As someone who would look at the reports, you might want to have a comprehensible report that you can look at and immeditaely know which form was submitted. With form IDs wouldnt you have to go back and check which form ID is assosciated with what page?