The last few days I have been trying to resolve a bit of an issue, a big issue in fact.
I have been looking everywhere on answerhub, the forums, google has given me many pathways, none of which have a solution.
What I am trying to do, is simply compile my 4.9 version of the project, with zero changes from my 4.8 version.
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First, to make sure my 4.8 version is working, I compile and it succeeds, no problem.
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Then I go to the Epic Games Launcher, launch 4.9, and open a copy of my 4.8 project in 4.9.
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I then attempt to recompile, and I get a whole plethora of errors and other problems, but when the compile fails, the reason is unknown, which isn’t very helpful. But I assume that the errors to do with being unable to find different classes and meshes etc.
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I attempt to “Fix up redirectors in folder” on the content folder, which takes a few minutes, but then appears to remove a few things.
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I do the fix up redirectors in folder again, which appears to do nothing now.
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I then try to compile again, still I have a plethora of errors, most notable are the “Can’t find file for asset” errors.
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I then look through some of the blueprints and I am finding that nearly every blueprint cannot be compiled, nodes are disconnected, behaviour tree’s cannot find classes, even though they are definitely visible.
The only way to fix these errors, is to go through every single blueprint it seems, find the errors, which in most cases are “cast to” nodes that have now become “Bad Node” due to classes that can’t seem to be found.
I have a LOT of blueprints and there is no way I want to go through every single blueprint to find these little errors.
And i’m not even sure that by going through all of the blueprints, that the problem would be solved.
I even attempted to compile the VR shootout demo, and the compile failed with unknown reason on that too 
I have made blank template projects in 4.9 and compiled those with zero problems however.
I did try to delete blueprints one by one, to narrow the problem down, but since almost all of the blueprints had missing references in them, I had to delete all blueprints before it would compile. And since my game is all blueprints, I pretty much had to delete the entire game before it would compile, lol.
I just attempted to copy the entire contents folder to a blank project, still no success.
I then removed the project, created another blank one, copied contents across, went into any of the blueprints that I could find that had errors in them, re-compiled. Some had errors because another blueprint wasn’t being recognized, so the Bad Node’s that kept showing up were on cast to nodes that previously worked. I had to go to the class blueprint that I was trying to cast to, and click compile again, even though it had zero errors and was already showing as compiled, and then had to go back to the project with the bad node and re-do the cast node.
(if that makes any sense).
Still, even by doing this (I found about 4 blueprints so far that had bad nodes in them), the build compile failed
