**UPDATE: *** SOLVED FOR NOW *****
In short, after experimenting more with hex editing, I realized that it was not reliable enough for me for many projects I tested. Your mileage may vary.
The problem was that the multis were not being properly loaded up if I had used Kontakt 4 as part of VST Instruments, as opposed using it as an Instrument Track. I could get Kontakt 5 to load up in Kontakt 4’s place, but too often the multis would not properly load. I could not figure that out. I had much more luck if Kontakt 4 had been used simply as an Instrument Track. So this hex-editing technique may still work for you, depending on how your projects are set up.
So, bottom line, I believe that the way Kontakt 4 multis are stored inside the Cubase 5/6 file are different to some degree than the way Kontakt 5 multis are stored. Or maybe I just missed something obvious and couldn’t figure it out. Thus, the “fuseon” hex editing method was not reliable (at least for me) for complex projects with complex multis.
If I had more time, I might experiment more, but since I had unreliable results except for the simplest of Kontakt 4 projects, I have given up on the hex editing method for now.
So, what did I end up doing?
I backed up my DAW carefully (every drive imaged), then I went ahead and installed Kontakt 4 on there. The installation went surprisingly smoothly, and so far has not conflicted in any way with Kontakt 5. I was able to get Kontakt 4 x64 (latest version I believe is 4.2.4) working just fine along side Kontakt 5 x64 (5.0.2).
Result: all my Kontakt 4-based projects are now opening up just fine so far.
So, if you want to save some time, I think it’s probably easier to skip the whole hex editing approach and just install Kontakt 4 on there. I’m not too happy about having to install an older plugin like that, but I will admit, the installation went more smoothly than anticipated and there appear to be no conflicts so far. So overall, it seems like the easiest/best solution.
Then, over time, I can rebuild/migrate projects using Kontakt 5 without any worries. I’ll have to be careful in the future, though, and remember that for the next DAW I build, I have plan for this type of issue far better.
I do think Native Instruments should have provided some better approach to this. Perhaps some tool or import/installation option so that we don’t have to install Kontakt 4. But then again, we’re talking about NI here… customer service hasn’t exactly been their #1 priority.
Alternatively, it would be really great if Steinberg had some sort of “replace plugin” tool within Cubase. That might have done it… except perhaps for the multis issue.
Finally, I have not tried opening any projects that used Kontakt 3 yet… I’ll play around with this current configuration first. So at least for now, it looks my primary concern is SOLVED.
Thanks again for your responses/help. If someone does decide to jump into hex editing, please do post an update. Would be good to know whatever tricks you come up with. I still think that theoretically, the hex editing approach would be a great solution (especially since you could create a simple batch processing/conversion script), but I just didn’t get it to work well enough.