It reaaaaly just depends. What you stated is somewhat of a sticky decision to make. I don't think there is anything wrong with using old warriors and monoliths. The only issue I see is if you decide to have different warriors with different guns. Unless you convert those. And possibly if the new monolith is a lot bigger. Destroyers appear to have a sizable body size increase. As a necron player with a ton of stuff ranging from the old pewter 1st gen to current....I plan to activate my GW brain chip and dive head first into the new range.
If the price is the issue, go slow. Just buy a couple kits at a time and build and paint to completion before moving onto another new kit from the new range. If you are in a rush to game at the same time, maybe use some of the old models as proxies until you can replace them with the new ones. My plan is to buy all of the new stuff and play with the old. Over time I will will paint and replace the old with the new. The models look too cool not to. I painted my original army in Sautekh colors, and plan to do a new one in the silent king's dynasty.
But for safe models I'd say:
Tomb blades (though I'm hoping for a re-boxing with bigger bases)
canoptek wraiths (please rebox with bigger bases)
ghost ark/doomosday ark
cryptek(new one on spyder bot)
Basically anything not finecast and has a green rod. Although it is completely up in the air on what will happen to the finecast characters (obviously besides szeres since he has a new model coming). I am super curious if either nothing happens to them or if a few get cut out...or if they all go away(unlikely). So I would wait on those if you had a character in mind. I have a feeling the old necron lord and cryptek finecast might go away in the new codex. Replaced or just simply removed. Same could happen with the C'tan. As iconic as the nightbringer and deceiver are, they have a chance of going away too. I am really hoping the silver body on the supposed void dragon model is a main base that could be used to make another type of C'tan.
Forgeworld would also seem safe at the moment.
Edited by Ahzek451, 27 May 2020 - 03:33 PM.