You can try directly upgrading to i-MSCP 1.5.3 after copying all necessary files to the new server. You can search the forum for migration relating questions. This isn't only a question of SQL tables.
If this does not work, you have to upgrade i-MSCP on the old server (or a copy) first, as Ubuntu 18.04 is only supported by i-MSCP 1.5.3's latest build.
If 1h downtime is a problem, you really should test upgrading on a cloned server and from start applying patches and upgrades to critical software regulary.