This.As a fellow developer I understand the plight of having to maintain old software, so migrating to a new app in itself isn't a bad idea.
However it seems like the hybrid app isn't an app at all, but simply the browser version that's been fitted to a phone screen. The UI is not at all optimized for mobile screens, zooming around is awkward and not good UX design, and some buttons just are naturally so small that even on max zoom they are a pain to reliably click. The benefit of the old app was that it actually functioned as an app - it was clearly designed with the small screen it would be displayed on in mind with decently large font sizes, minimal fluff, and UX that made sense on a touchscreen.
I think that you're going to need to put a bit of effort into accessibility and actually designing a mobile-friendly interface for the hybrid app that provides tangible benefit over just opening your site in a browser, because as it stands I think I'd much prefer staying on the classic app despite it becoming unsupported.
The classic app needed work, it needed better functionality (missing features etc) and it needed something done with the jumpy/glitchy forums etc.
What it didn't need was to be ditched for a port of the web browser that has 0 optimisation at all.
Things I like: Er the forums are more readable on the app. I can see advisors/ events.
Things I do not like: Landscape only. No mobile optimisation. Buttons are small, unscaled & fiddly, hard to press and not responsive. The scaling is poor & keeps resetting, and doesn't let me zoom in properly anyway. Posting on forums is horrible as my keypad in landscape covers most of the screen so I cant even see what I am typing.