Ah, alright.
There's nothing actually stopping you from doing this — it requires two things:
1. either a third-party licensing server (and thus some SSO auth system — but just require Apple's own SSO for it and it'll still be a clean-ish workflow) to share/sync the transaction status from one app to the other; or a local Group Container plus logic in each app to write the transaction statuses fetched for the given app into the group container for the other app to read
2. never charging for the app as a whole, but instead breaking your app's pricing down into a set of IAP-purchased feature entitlements (whether charged for individually, or as a bundle, the important part is that each entitlement has its own price.) Then, making the new version just a superset of the features of the old version — and so, when you're buying the old version, you're buying features A+B+C; and then when you're buying the new version (with the app being able to see whether or not you've bought the old version), new customers are buying A+B+C+D+E, while existing customers are buying D+E.
---
Note that there's an even easier way to do this (and I think this is the way Apple would prefer you do this): don't release V2 as a separate app from V1.
Instead, have V1 auto-update to a v2.0.0 release — which converts the V1 app into a launcher with an "edition" (major version) selector. Either compile in both the V1 and V2 codebase into this app, or better yet (for download/on-disk size), package separate V1 and V2 "engines" as executable DLC packages, submitted to Apple for review along with the app, downloaded on-demand when the app needs to run them.
With this approach, the app would either start up the first time still within V1, and allow/offer people the option of "seamlessly upgrading" the app to V2; or the app would start up with an "edition launcher" UI that allows people a choice. (And either way, you could offer the ability at any time to freely switch between V2 and V1, re-launching the app with the other engine enabled. Like dual-booting Operating Systems, but at the app level.)
Here, you could charge for the V2 "upgrade" ahead-of-time, before allowing the user to switch over to the V2 engine; or you could allow the user to switch between V1-fully-licensed and V2-demo modes (or even between V1-demo and V2-demo modes), where purchasing for each edition is separately available within that edition's UX.
The expectation here is that all the user's existing feature entitlements would keep working as long as they continue to use the V1 engine — as you said, the V1 engine was a one-time purchase, and so even with this edition-launcher abstraction introduced in v2.0.0 of the app, V1 itself should still keep working for them forever.
The benefit of doing this multi-editioned-shared-app approach, together with IAP feature entitlements, is that V2 can inherit some of the V1 entitlements, and then simply charge for the V2-novel entitlements. So V2 gets discounted for V1 purchasers inherently, by the fact that by buying V1, they've already bought half of the components of the V2 purchase-bundle.