After another reboot of each DS, the SVS agent is now getting downloaded to the client.
However, the AClient still is not getting applied, at least from one DS. I've done further testing and found more info which may help give clues to the cause. As mentioned previously, we have two servers running DS (I'll call them DS-A and DS-B). DS-A is also running SQL 2005, which is hosting the databases for each instance of DS. Deploying the new version of AClient from DS-A works fine. The problem appears to only affect DS-B. More specifically, it seems to be the "File Copy" job that's not functioning properly. On DS-B I have some older jobs which simply copy some fonts to the workstation. If I run these jobs, they work fine. And if I add a task to one of these jobs (ie, to copy a text file to the workstation), that too works fine. But if I create a new job to do exactly the same thing, it doesn't work. I can see that job queued, but it doesn't run (basically the same as when trying to run the "Agent x86 Update" job).
Repeating the above File Copy tests on DS-A work fine.
As far as I can tell, the AClient on the workstation is connecting to the DS. If I open AClient on the workstation, it has the correct details for the DS. There are no DNS or firewall problems. If I reboot the computer, I can see it grey out within DS and then go green again once it's back up.
I've also just created another new job on DS-B to reboot a workstation. This also does not work. But if I use the existing "Restart Computer" job within "Misc Jobs > Power Control", that works fine.
So, based on my testing so far it would appear on DS-B, new jobs aren't working but older ones (pre-update) are fine, assuming the "Agent x86 Update" job is a newly created one as part of the upgrade. It's certainly configured itself to copy the latest version of "altiris-aclient-6.9.375.X86.exe".
Does this give a hint of what could be causing the problem?
P.S. I'm only using AClient, not DAgent.