I was saying that, if you have installed an FS9.EXE with a nocd patch, the FS9.1 update will fail, because it will not update any file that is not the original version, as for the FS9 CD. This is also valid if any of the default .DLL from Microsoft in the MODULES folder were altered, but this is far less common.
So, the correct course of action, if you have a nocd patch, is to FIRST restore the original FS9.0 EXE from the FS9 CD (use a archive viewer, like 7ZIP or Winrar, to look inside .CAB files of the installation CD), THEN run the 9.1 update.
And, of course, if you want to use a nocd patch with 9.1, be sure it's a 9.1 nocd patch.
However, our installer doesn't really care which version of the .EXE you have so, even if you put a 9.0 nocd over an existing FS91 installation, our installer will not complain BUT, it might just be not reliable to run an FS9.0 EXE with a whole set of 9.1 updated .DLL.