It will be a similar change to Age of Sigmar to Age of Sigmar 2.0 - essentially the same game, only with the FAQs that gave come since baked in to the core rules, and a few tweaks here and there. There won't be any radical changes, just a slight refinement, an exercise in tidying up. It's absolutely the best way of doing things, in my opinion.
40k (and to a lesser extent AOS) are different beasts to every other tabletop game out there, because of their popularity, the sheer number of armies and units, and the relentless release schedule adding more and more all the time. There are so many combinations that players can take advantage of that GW keeps having to play wackamole by releasing FAQs to stop the overpowered lists from ruining the game. As a result, even though it's only been a few years, there has been so many adjustments it's actually quite difficult to get a handle on exactly what the rules are now.
Personally, I think the only sensible way forward is to move to an online living rule book, and the same for all the codexes. So you have the rules as an app, and when there's a change, the app notifies you and updates. This could be what they are moving to with the new app, I expect. Even this has problems - who wants to be setting off for a game and find that the rules you've learnt have changed? You don't want to buy multiples of a unit and find it suddenly becomes less powerful. But that's already what's having to happen, it's just that now there will be less paper involved. Anyway, what were you doing buying multiple units just because they were powerful?
The playerbase creates the need for these updates because a significant proportion of them are focused more on winning than enjoying the game with their opponent. With so many available combinations, it's inevitable that someone will find the broken combination. Then the next tournament is full of that list, and another FAQ becomes necessary. This is a better way for GW to handle their game than the previous policy of leaving the broken stuff broken and saying that Warhammer is not a tournament game. It's always a bit of a shame having to balance for the win at all costs tournament players, but GW have realised that this competitive mentality is shared by a lot of the player base. I'd prefer it if everyone wanted to play to enjoy the spectacle and the narratives that develop on the table, but then that's just me wanting everyone to share my opinions, and that's the sort of thing an arsehole does. Besides, even as I'm enjoying the spectacle and narrative I'm hoping my dice roll sixes, and that the strategies I've planned work out, so I'm a hypocrite.
It sounds like the cost to buy-in to the new edition will be exactly £0 anyway. Download the app and you have the rules. You can still use your 8th ed codexes. If the app is the same as the AOS one, you'll even get all the rules for each individual unit in that. You just won't get the army-wide rules, strategems, etc. There's going to be another big box, and if it's anything like as generous as the last new edition one, that's a good thing. I'm looking forward to it.