If it is an analogue tuner no point in retuning as it wont pick up a signal.
You can retune a digital tuner
So it depends what type of tuner is in the the device (as Chuck says).
A VCR will almost certainly have an analogue tuner in it so cant be retuned. You can still WATCH VCRs but not record using the tuner in the box.
A DVD recorder MAY have an analouge tuner or a digital tuner (check with the documentation that came with the DVD player). An older DVD player will have an analogue tiuner, a new one a digital tuner.
Sky is already digital (unless it is a VERY VERY old Sky box) so wont need retuning. Sky gets is signal from a satellite not an aerial anyway so does not need a retune.
Thank you so much for your answers, so sorry for the late reply.
I have tuned my Sony DVD recorder in, but regarding the Liteon I have not bothered anyway, because I no longer record on it, but keep it to play back DVDs and it's hard drive content.
The same applies to my VCR, I only use it to transfer old tape content to DVD discs.