How did you know in advance? Just mass accumulation of knowledge over the years?
Yeah, in 15 years of programming, there are only two reasons I've ever seen for not being able to open an old version of code. One is if the old VI was written with something too old to convert. The other is a missing block diagram. Since the original was written with 7.x, I knew it had to be the second reason.
You can't open a VI in a newer version of LabVIEW unless the block diagram exists. The block diagram is required to recompile it for a new version. As I already said, you have to find the original. There is no other option except to rewrite it if you know what it did.