Thanks everyone!
JasonLion, you're giving me hope that's a threaded coupling. I hadn't noticed that. The left side looks glued so maybe it's only threaded on the right. Not sure why they would have done that, but if it comes apart, that sure makes things easier. I'll have to try that while also having a plan "B" in case it doesn't come apart. I'm afraid of something breaking when I open the union, or not being able to reassemble it (1980's era pipes).
zea3, the union nut opens to the left (away from pump). Would've been nice the other way. The connections from the 4-way to the valves are glued.
jodeci77 there's just under half an inch of pipe there. I'd be concerned that's not enough for a reliable cemented connection to new parts.
JamesW and Iciee, you've both offered a good plan "B." I had to Google "PVC fitting extended" but I see how that works here.
So I'll tackle this when I have a whole weekend just in case something goes wrong, and I'll report back. I'm hoping next weekend will be OK.
Thanks again to all of you for good advice and keen observations!