Recording issues with using a mixer as interface

Ah, that's the one thing you can't do - load a track to record over in Multitrack. This goes back years, and is really a result of Adobe paranoia about being sued for consequential loss if somebody accidentally or otherwise records over a track that's actually wanted. If the software allows them to do that, they think that this would somehow make it their fault. In America at least, they have some grounds for being worried about this - it's happened in other industries.

But what you said isn't quite clear. If you want to play the previous track whilst recording another, then you have two options; either move (drag) it to the next track, or turn off record on the first track, and set another track up to record on instead.