> I have a piece of source code (originally developed for IB 6 then FB
> 1.0.3 then FB 1.5) that is calling ISQL.EXE with a -merge_stderr switch.
>
>
>
> In FB 2.1.1 now, it no longer recognises this switch.

You can print list of valid switches when you call isql with invalid switch (e.g. isql -?)
There you can see
-m(erge)
i.e. use either of -m, -me, -mer, -merg, -merge