While an in place upgrade from 16.x to 18.x (also known as a dist upgrade) is supported by both Ubuntu and Percona, it is not always guaranteed that things will go smoothly. So regardless, you should always have a full backup of your database that you have tested and verified to be good. If it were my data, I would build a second instance based on 18, migrate my data over, and only after I have verified that my data and application function well with the 18 instance, get rid of the 16 instance. This is just my opinion, best of luck to you whatever path you choose.
–
George O. Lorch III
Director of Server Engineering, Percona Server for MySQL, Percona XtraDB Cluster, and Percona XtraBackup