I don’t usually post here, but as someone who only knows a bit of blueprint I was devastated, I didn’t understand what was going on everytime I tried changing my 4.27 project to 5x I was crashing everytime I opened something relating to a major characterbp. Upon googling, apparently it has something to do with structs. I’m sure manually modifying the structs might help but I wasn’t trying to do something as tedious as that. Just in case any googlers come by this thread, here is another possibly more user friendly option.
First before even changing version or migrating, make sure everythings good; use tools like fix up redirectors, validate data, refresh all nodes. Search your entire content folder for structs and note which asset uses these structs. For me, I had about 10 marketplace assets that were tied to these structs.
Next, get that list of assets and simply install the 5.x versions into your project, replacing the old ones. This will get rid of any compatibility issues that a 4.x version of the asset will have in your 5.x project and effectively replaces all the problem structs with working ones.
Now open the project , validate data and fix redirectors. Now everything should be able to be opened without crashing. Instead of messing with structs you’ll have to go into each asset that used the struct and fix up error messages. For me this is easier since I know my way around the blueprints and not the structs.
Hope this little fix from some newbie dev helps.