Colonel Panic
Thanks Brian, valera, anticapitalista and vitforlinux for your replies. I agree with valera in particular that upgrading antiX from stable to testing should be avoided (or at best carried out with extreme care); I have no plans to do it myself.
It’s not often I need a package that is more recent than stable has anyway, and when I do I can just unpack it to /opt and run it from there (I usually use midnight commander for this).