Depends. If your project is valid at the start (meaning it includes the cache lib!) then it is straight forward.
Worst that will happen is your symbols get rescued. And this only happens if your libs changed. Which they will have if you run the official libs as there was a massive reorganization. (You can keep on using v4 libs for this project but i kind of fail to see the pay-off for the extra work needed.)
More details: Converting KiCad version 4 projects to version 5 (Remap a project)
And in general: There is a bug in v5.1.2. It sometimes shows symbols as question marks until you restart eeschema.