Thank you very much this has been very helpful! But I would still like to know if I could switch the server type from Spigot to PaperMC as suggested in the article and how to safely update the server to 1.14.3 without deleting the current world.
EDIT: Just read the uploading a world support article and I'm afraid that our current world might be too large as we've explored all 3 worlds (Overworld, Nether, and End) but I do not know for sure.