As long as the .uproject is built for 4.14 (if you open it on engine version 4.14) then it is already a 4.14 build. I would try rebuilding the binaries, opening the project, saving, and then see if it gives you the same errors.
As long as the .uproject is built for 4.14 (if you open it on engine version 4.14) then it is already a 4.14 build. I would try rebuilding the binaries, opening the project, saving, and then see if it gives you the same errors.