It would've been great for the 16.6 upgrade to configure itself to just work properly without its end users having to go through all of the hocus pocus. I don't really understand the need to switch between versions, either. Ideally, all of the new versions should be backwards-compatible. I know from my limited experience with OrCAD that this probably isn't the case, hence the need to switch versions. Since my company started using OrCAD recently starting with v16.5, hopefully, I won't run into the need to switch versions often.
I have learned never to download an ISR or upgrade of any kind while I have current projects that I'm working on, because the chances are pretty good that the new software will cause an error that will stop my work in its tracks. I waited until the day before new year's weekend to install the 16.6 upgrade for just this reason. I figured that if I ran into a problem with it on Friday afternoon, I'd have the long weekend to fix it. As it turned out, it worked just fine on Friday. I was relieved! But, Wednesday morning, I started gettting the *.dll error, which led me here. Tech support didn't have the answer. I figured out to delete the registry data for brd files myself. It might have been the "dirty" way to get it done, but it worked. This time. Next time, I may have to download one of those "unassociate" utilities that oldmouldy suggested.
I know that many of my problems with OrCAD are due to my position on the learning curve; I have a lot to learn. However, I think that much can be done to make the installation and implementation of ISRs and upgrades more seamless and user-friendly.