I literally just went through this, and tried to do it the hard way. I re-created all my structs and enums in C++, then renamed the BP ones to *_Old so I could find them, and set about changing them.
This was a mistake.
Every single pin, every single conversion node, retained the old types.
Then, when I thought I was all done, I had to rename my enum indices and do it all again. I couldn’t find every single instance and my log was full of errors. Blueprints just retain a LOT of data that you would think would change automatically. It doesn’t.
That’s when I was shown the wonder of Core Redirects. You can pretty much use the examples from that page and just change the values. They go in Engine.ini and when you load your project it’ll rewrite every matching blueprint type to be the corresponding C++ type instead. Works for classes, enums and structs. Give it a go.