You’ll be able to keep your original panel, just means you don’t have the new dedicated buttons that were introduced with V4.
The case will need some more cut-outs for the Core32 interfaces and the Ethernet module, if you’re interested in OSC.
I upgraded from V3 to V4 like that (no Ethernet though), it’s working well.
But I’ll redo panel and case anyway (kept the wooden breadboard thing, see my avatar), so I’ll add some buttons and redo some wiring in the course of that.