Not sure where you heard that, but it's not true. i440fx works perfectly fine when moving a VM from one set of electronics to another. Not sure where you got the impression otherwise.
I think you're misunderstanding what I'm trying to say. I use Q35 Windows VM's currently. I need to migrate them to my new hardware. My concern is that the VM XML will be different if the XML is generated based on the hardware of the unRaid server. If the XML is the same regardless of hardware, then this would be a non-issue. If not, I'll be in trouble if Q35 is removed as I won't be able to properly generate the XML.
If you change the motherboard of your computer, Windows wants you to call MS to reactivate the license (if you're not signed in with a Microsoft account). This applies the same with virtual motherboards like i440fx and Q35. If you've associated your registered copy of Windows with your Microsoft account, you'll just need to resign in once you change the gear and all is good.
All my Windows 10 VM's were upgraded from Windows 7 or 8 and from what I understand, it is an OEM license and can only be activated on one machine. A server hardware upgrade or change to i440fx would kill my activated licenses with no recourse other than to buy new licenses. So keeping Q35 would be helpful in this case.