I get that. But my situation (i think) is a little bit different. This is what i did in more detail:
I started by setting up a new machine. My provider only offers preinstalled 14.04 or 16.04. I went for the 16.04 and upgraded to 18.04 straight away.
After upgrading the fresh machine I started the fresh MIAB installation (I have been following the Mail-in-a-Box version v0.40 and moving to Ubuntu 18.04 guide) and everything goes ok until the end when I get the “Waiting for the MIAB management deameon…” message which goes into a loop…
Is this situation equivalent to ‘dist upgrade for MAIB’ that you refer to?