Let’s go step by step.
Your questions:
What would be the best way to do that? Do I just image a new SD card with umbrelos-pi4.img, plug it in, and power it up?
Yes. In theory, that would be the most practical and comfortable way to do it. It shouldn’t give you any problems if you haven’t altered/modified anything in your current umbrelOS.
Will that approach take advantage of the system’s currently up-to-date blockchain, or will it have to entirely resync?
In theory, whatever you have installed (stored) on your drive will be respected and recognized by the new version you flash to the SD. There shouldn’t be any problems with that if you haven’t altered/modified anything in your current umbrelOS.
Or should I erase the blockchain’s drive to make sure it entirely resyncs?
In theory, you don’t have to do any of this.
OK. We’ve already talked about the wonderful worlds full of princesses, unicorns and blue princes. Now, let’s talk about the harsh reality.
Don’t trust the fucking theory. If something can go wrong, it will go wrong. Especially with umbrelOS. There’s always some problem with every update/upgrade.
Personally, I always have a backup of the blockchain and I can recover everything easily and quickly if there’s a problem.
In your case, it seems that only the blockchain is important so I assume that the important thing in your umbrelos is the bitcoin node. So if something fails, all you have to do is format your disk, reflash the SD and start from scratch by downloading the blockchain again. It’s not a big problem really.
But, well… try the first one. Maybe everything will be fine.
It might be worth taking a look at this: umbrelOS- Good Practices