I create fillable forms starting in Word, rather than Illustrator, but I think the same method should work.
- I have a fillable form where I need to update the content.
- Make the edits in Word (or in your case Illustrator) and save as a pdf.
- Open the form pdf in Acrobat Pro, go to Tools > Organize Pages and select the page(s) I need to update.
- Click on "Replace" and replace that page with the pdf I just updated.
- Once the page is replaced, click out of tools and your form fields should be still there, with the underlying page all updated.
- You may have to go to "Prepare Form" to reorganize, resize, move your form fields, etc. but it's a lot less work than trying to make big changes in Tools > Edit PDF.
I know this isn't what you are asking, exactly. It'd be nice to keep everything in Illustrator, but I hope this gives you an alternative if you can't get Illustrator to behave nicely.