When I tap on one of my craft buildings, I appear to get an option to convert it. It gives a option of another iron mine - which would be extremely useful. However, if I try to select the iron mine, nothing happens. It just has an option to cancel.
Is there something I’m doing wrong? Or is there a glitch?! ! Screenshot_2018-10-18-07-48-52|250x500
I think I might have answered my own question. Clearly not awake yet. That’s not an iron mine there, but the original crafting building. Guess there’s nothing for me to convert it to 
At this time, buildings can only be converted to a barracks at level 10(?).
This allows you to level up troops.
If you have a barracks you can convert it to the original building and turn something else into a barracks, but that’s it.