Not sure NVA have a formal 'upgrade path.'
What I mean is, you get more in the power amps - if the speakers and rooms need it - and the preamps get more versatile firstly, then 'better' with stepped attenuators. Maybe the speaker cables get better too, I can't say as I have LS2 which is fine for my humble downstairs-stereo needs and LS5 upstairs which really is impressive and slap bang in the middle in terms of price.
I've spend a fraught half hour looking up UK audio dealers (started off as something else and became a game of 'Consequences'
) Looking at the crap for sale costing tens of thousands - it's 'all' casework I tell ye - I feel traumatised and also very sad to be out of it in all honesty.*
* Bottom line, NVA is a fair price for a great basic product design. I'd have been more inclined to look at good vintage pro amps for the second system - I mean, you can buy a Crown D-75 (not the supposedly inferior 75A, and the 75 was of the first generation using more up-to-date internals - 5% resistors and so on) for well under a ton and it has half decent gain controls on it too, so doesn't need a preamp if you just have one source, or merely a switcher if you do...
Hanging on in quiet desperation is the English way...The time has gone, The song is over, Thought I'd something more to say...