[Quickform] Change values of a hidden field

Hi, I am using Quickform to realize a form on a page. I like to use a hidden field to pass a value of how the data beeing passed should be treated in the skript afterwards. It seems that there is no way to change the value of this field once an intial value has been set. I now tried for one day and I am on my wits end. Could anybody please give me a hint what to do? Thanks in advance. Thomas