Yes, because the content of the combo list are prepared in the view file, and sent to the JForm field in the template.
Generate without Ajax, and :
1- keep the code wich is preparing the list (displayXxxx())
2- Keep the few line in the template (Dynamic initialization)
3- Keep the parameters in the XML file (dataKey, dataObject, ...)
Ajax call needs the ajaxContext property to show,
and the Vars sent to it (Dynamic init)
But in your case, it becomes tricky because impossible to switch XML paramters depending on condition. You may have to create 2 different fields with the same name. It is not very clean and could raise some issues.
How I would do :
Keep an ajax field for your XML,
and write the conditional normal combo with a hardcoded JDom call in the template (not using XML field)
This field should recieve : dataKey, dataObject, list, listKey, labelKey, (eventualy groupBy array)
Hope it helps.