grizzler wrote:I know that must sound a bit scary to someone familiar with Linux distros and I certainly wouldn't want to do something like that to my Linux box, but I can't remember how many times I have had to perform a hard reset on my old RISC OS machine over the years...

Hahaha, yeah, it does sound a bit out of my comfort zone, but I want to make a concerted effort to learn how to use a new-to-me OS (well, I used it briefly back in school twenty years ago, but I hardly think that counts

), so I want to get to grips with this sort of thing.
However, the 'normal circumstances' I mentioned only apply when the new ROM is in the same range as the old one, i.e. both are 5.1x or 5.2x. I assume you want to move from 5.19 to 5.21 and unfortunately that's a little more complicated. A 5.1x ROM expects a $.!Boot.RO510Hook structure and a 5.2x ROM needs the corresponding $.!Boot.RO520Hook one as well. This is available in the HardDisc4 image on the RISC OS Open site.
Yeah, that's the move I want to be doing! Typical, eh?
My understanding of RISC OS is, at this point, that of a complete beginner, so I'm afraid I didn't understand a lot of what you said in the above quote (well, I get the bit about ROMs in the same range, but that's about it!)...
The RISC OS Open forum has several threads about handling this upgrade, but I don't think any of them mention the peculiarities of doing it on a Pi. This is relevant as the Pi image has a number of extra files which aren't present in the normal HardDisc4 image.
As far as I've been able to find, none of them do mention this. I know about the differences, vaguely, but that's where I've gotten stuck.
I saw a post on the RISC OS Open forum that suggested that the upgrade on the RPi version can be done via !PackMan, but I'm now unsure if this applies here...
Even without that complication, it's far from a user friendly process. Unless you're a bit of a masochistic nutcase like me, you'd probably be better off reflashing the card with a new image and copying things you introduced yourself using a USB stick/drive over a powered hub or flashing another card and copying things between them (in theory 'hot swapping' cards under RISC OS is possible, but my Pi's USB stack sometimes crashes when I do that).
I'm not a masochistic nutcase by any means, but I've found that I learn new OSes best when trying to do things that are considered a bit tricky by many.

Even if I'm not successful, I tend to learn well that way.
If you are interested in doing things the hard way, I could describe what I did to upgrade my 5.19 installation to 5.21 without reflashing the card. Mind you, I currently have an ADFS formatted USB drive connected which contains most of my setup and only use the SD card to provide the ROM image. So I have to translate this to a 'pure SD environment' which may complicate things further.
I would be very interested indeed, please, but I don't really want or like to impose!