Datatable with changing AJAX Source

Hello,
I'm using a Datatable to retrieve certain information via "sAjaxSource".
In my application I need to change the source URL, I achieved this by calling myTable.fnSettings().sAjaxSource = "someNewUrl" and fnDraw(true)

The fact is that the "new" returned data have a different number of rows (column number can change multiple times).
I do rewrite the <thead> of the Table using an AJAX request which returns the new correct Heading, but my Datatable breaks afterwards. The <thead> is correct, the AJAX response of the Datatable returns correct information (for example 3 columns now instead of 5).

Debugger: http://debug.datatables.net/elipeb

At the Debug information you can see that the <thead> is still the old one and row has less columns (null-values)
I'm using jQuery 1.9.1 (+Migrate Library) and Datatables 1.9.4. My application is on a nonpublic development machine.

What I need is a function which parses the <thead> again, I searched in the documentation but didn't find anything like that.

Any ideas are appreciated!!!
kind regards, peipst