I'm afraid that's pretty much it. You'll also have to be mindful of attribute naming conventions that might differ between PostGIS and SQL Server as well as behaviour with regards to null values.
Hi Tobias,
There is a right-click in the navigation tree on a reader or writer that says "Update". That will allow you to replace the reader or writer with a totally different one and still keep all the connections intact. So I'd explore that.
Now, it is true that the workspaces themselves are just text. So if you want to be really brave...what you could do is upgrade one reader and then compare the resulting saved .fmw to the version before it was updated. Note that you only need to care about the XML part -- the FME "language" further down will be regenerated by workbench when you open, do anything (like move a transformer) and then save.
So..you could then try to use text processing in saw awk or perl and make the changes to the XML portion in an automated way. Then open each workspace and save. I'm not sure I'm recommending this or that it would be worth doing...but if there are enough workspaces maybe...
Or you could hire your teenage nephew to do the point and click "update reader"...
Good luck!