I recently opened some of my work from 2009, and I have a mono master bus. I do not know whether this is a failure to import correctly or something I did wrong when I recorded the audio 5 years ago. I can work around this or import the audio into a new session, but what I really want to know is how do I fix this?
Particularly as if I find more works that are like this they may be more complicated. A lot of my later work has much more done with automation trails and suchlike.
one can add and remove channels to/from every track or bus. The functions are accessible in the context menu of the port.
In the mixer window, right click on the channel strip’s input and/or output connector button. -> Add audio channel.
The same is also available in Menu > Window > Audio Connections for every port.
I guess the 2009 works are Ardour 2 projects…? Have you opened them now in Ardour 2 or 3?
I remember that A3 has problems with A2 projects (I remember this has been a forum topic in the past). But it’s absolutely no problem to use A2 and A3 on the same computer and work on the old projects with A2. 
Michael