There used to be a page that listed ECU interchanges.

As I recall, the page said that the Convertible had a faster clock rate or better processor or something in it and that the Coupe ECUs could not be put in to a Vert, but a Vert ECU could be put in a Coupe... Or something like that...

Does anyone know what I'm talking about?

 

The only differences I know of between the S4 NA coupe (N326, N327) and S4 Convertible ECUs (N338) are minor tuning changes to account for the vert's increased weight, and the fact that the N338 will apparently work just like a TII ECU if it's hooked up to TII sensors. Hence the number change to N33x.

I've also used an N327 ECU in my vert (when it was NA), and didn't encounter any problems.