Hey Bernhard, I am not immediately aware of a way to accomplish precisely what you are asking. I can think of two close alternatives.
1) You may could use a "Level-2 MATLAB S-Function" block and programmaticaly change the MATLAB S-Function to which it was referring (the Dialog Parameter "FunctionName").
set_param(handleToBlock, 'FunctionName', 'newFunction');
This would require all the S-functions to have the same input and output properties. You can execute the command "sfun_demos" to view several examples of using S-functions.
2) You could use a "Variant Subsystem" block. Inside it could be several subsystems, each with a MATLAB function block inside. You could then programmatically update the conditions to vary which subsystem was enabled. You can read more about variant subsystems at the documentation linked to below.
