Is it even possible to go from a Valve-delivered SHIII 1.4b to GWX? I tried copying the installed game to a separate drive, as I didn't want to risk compromising the stock version, and then applied the GWX upgrade to the alternate location. Installation of GWX was uneventful, but on attempting to start the game I get a load error notification (A:0000065558 or something such).
I suspect a registry error, or the fact that Steam doesn't recognize the modded version as its own.
Thanks. I followed the instructions below to the letter, but as it is a Valve-delivered copy (1.4b) I cannot select the installation folder myself. The SH3 exe appears to be slaved to the Steam shell: a simple copy-paste of folders does not disassociate the game from Steam, or so it appears.
1. Install a clean version of SH3 in the C\Program Files\Ubisoft\SH3
2. Patch it to 1.4b
3. Copy the whole folder and paste it whenever you want the new installation to be. After that rename it to whatever you like. Let's say GWX.
4. Now put the multiSH3 into the GWX folder and run it. When asked put GWX as the default directory where your careers will be saved.