Can I have, say, 3 versions of Renoise on my system all at once (eg. 1.7, 1.8 and 1.9)?
Haven’t upgraded to 1.9 yet … 
simply install it on a different location. each version has its own application data (f.e. in Windows is something like: c: \ Documents and Settings \ yourusername \ Application Data \ Renoise \ V. 1.9.0)
thanks!
[edit]
Oh and what file(s) do I copy to transfer all my settings (colors, folders, …)
nvm, they’re being transferred automatically.
Renoise takes care of this when launching new versions the first time, so all you have to do is start up Renoise…
Compatability issues with older versions (though very minor ones!).
Behavior between <1.9 for certain pattern effect commands changed for instance.
If you don’t want to re-edit your old songs to change this behavior (because it can be adapter) but rather want to rerender the output, you can do this using the old version.