It looks like your original issue was that you had a ControlSet of WebDialog11, which is not yet supported in Web 2.0. As to your question, I recommend rebuilding but many others have converted with varying degrees of success.
That’s my recommendation as well. Similar to what I’m doing with some new os Updates. I usually just upgrade but after a few releases, I do install from scratch. That’s what I probably will do with Big Sur. Web 2.0 is for me such a big change, that I personally believe it will cause me less headaches to start from scratch and “migrate” things by copying code manually and creating the controls for instance from scratch.
“And too big to fail” projects will just continue to run on web 1.0 … customers are informed that bigger changes beyond the next 3 months will be costly and are not recommended as we have now a better solution in place. If they are willing to invest, we will migrate, if not … bad luck. I’m selling it as a major release from an OS. Most customers really do understand it. They are anyhow used to it. Changes in Bootstrap, Wordpress etc. are pretty much often causing a similar pain …
I think that’s the right approach. I migrated one customer. That was a small project, but I still started from scratch. He is happy as Web. 2.0 is by far faster. But it is faster as well because I changed some routines.
With a sole copy & paste I would never have noticed this “bad” coding. And very often the UI is anyhow the biggest work. On top of that I have many Web 1 projects depending here and there on @Anthony_G_Cyphers 's great Graffitisuite components.
Hence as long as he’s not ready yet, it is obsolete for me to even start thinking of migrating these web apps ;-).
But it probably all depends on the type of app you are running. If you only have a few buttons and your app is mainly converting APIs into something new the user can download, then it will be easier. If you have a lot of UI stuff it is more complicated and more things will be missing.