nvidia -> AMD failure

Last week on my main desktop machine I swapped out an old nvidia card for 
which the nouveau driver was giving me trouble, and replaced it with an 
AMD-based RX550 video card. Everything worked great.

So I did the same on my main server for the house, whose (different) old 
nvidia card was also giving problems (but different problems). I bought 
another, identical, ARX550 card. I figured the replacement operation would all 
go as well as the prior exchange had done. But nope. With the AMD card in 
place, at boot time the server's GRUB finds the grub.cfg file and then finds 
the kernel and starts to boot. Lovely. I get the usual message about loading 
initramfs ... and then nothing further ever happens. The disk light shows 
activity for a while, but nothing ever changes on the screen to show the 
system actually booting.

I had naively thought that simply putting in an AMD card -- especially a model 
that I'd already used successfully in a different machine -- was pretty much 
guaranteed to work; but obviously I was wrong.

Does anyone know what I might need to do in order to get past the "loading 
initramfs" step?

On the server machine, I have put everything back the way it was (i.e., it now 
has the old nvidia card again) which means that the home LAN is up and running 
again and enables me to send this e-mail out.

I know this e-mail is light on details, but I'm not sure what additional info 
would be useful.

Oh, both the server and the desktop are running up-to-date trixie.

   Doc