We use an environmental variable too to point to the configuration database. It's a pretty cool method.
Right now for your problem you might want to look into do a massive find/replace in file on your DTSX files. Obviously you'll want to test it out so you make sure you get the right string to find/replace. DTSX file are just XML text files so you can use the Replace in Files in either BIDS, Visual Studio or your preferred text editor if it has that functionality. I've done this in the past when we did a server migration and it worked out quite well. Again, just make sure that you test on dev copies to make sure you are replacing only the text that you need to - you can mess up your packages if you aren't careful.