how to rejuvenate old holly hedge

I reckon yes! Holly is usually as tough as old boots.
You may want to wait for further advice, eg whether slightly later in the year is a better time to prune back?
Mind you, if you cut back now it will have plenty of time to resprout before cooler weather.

spring is good as its going into growth mode and can cover the old scars quickish .if you do it now it will be open all winter and be doorment excuse my spelling

Been there - done that and did so in August. I literally halved mine in depth and height - about the same as you want to - and I did it in August as it gave it a chance to recover just a bit before the hard weather hit it and also I could not do so before as there were so many birds nesting ..and I don't think it would have been legal to disturb them. Yes - it did look a bit bald - but I had a trellis erected on the affected side and popped a few wall pots on it with polyanthus etc for colour as a screen ....and it has been so effective I have kept it there as a feature! Also - if that holly hedge thinks it is going to grow back through - it has another think coming...I am ready for it!! I would certainly do it now...I don't know where you are but up here on the NE coast of Scotland I got away with it!! Good luck!

spring is still a better time though snivel i was only saying lol

Sorry NP...I agree that Spring is GOOD ..grovel grovel.....but I am a softie when it comes to all my little birdies:~))
Anyway - I was too impatient to wait - and if you have a look at the pics I have just added - you will see that in under a year - you would never know I had savaged the brute!! It loooks as wild - but at least it is only 6' thick instead of 12' etc!

no your right thistle is hard as old boots lol