You could use a hidden form input element for the id.
As for the pre-filled data, where are you storing it anyway, i.e. you'd get it from there... Otherwise if they use a back button then encode it in that (maybe better to use an actual button in that case), however if they don't use your pre-formatted back link then the data is lost, so you may want to store it in session variables, however they'll only be able to fill out one form at once then (which is what MOST people will do anyway, but there's always one)