Update multiple fields in a form on change event


Good day.

I am going to attempt this problem I am having from another angle seeing
either I am doing something so stupid that no one wants to comment on
it, or
because I am just so lost that no one has seen or done things this way

I am simply wanting to update multiple fields of a form after one
event occurs.

I am currently using multiple observe_field calls on the same input_id,
however, I get a too much recursion error in the javascript when this is
I would like to use the update_element_function, but it appears to me to
only work on submission of the form, as the action in the element
says onsubmit().
I need the form to update once the user makes a selection from a drop
list and then populate values in 3+ other fields.

Has anyone done this before?
Is this not common practice in javascript and with forms?