I've got a simple question that may or may not have a simple answer - is it possible to tell the governors {or whomever} to upgrade/replace all improvements of one type into another - for example, I have schools, but I traded for research centres, and I really don't want to go through each one manually, etc.

No, not to a different structure type.

If you have plenty of resources then you could demolish all and set all the new structures to build.

If things are more tight then you may want to leave all the old structures in place to add their effect while you replace one at a time.... and it is tedious!