virtualbox.org

Apologies if I missed this. I have not found simple instructions for upgrading VB (as opposed to installing de novo).

A big reason for asking is that some software recommends removing an older version first, while some can deal with the situation gracefully.
A related question, noting some of the issues people have have with 4.2.6: is 4.2.4 a safer choice?

Running VB 3.2.8 on OSX 10.6.8 ; planning to move to VB 4.2.X. WinXP is sole guest OS

Thanks for any hints/tips!

The general rule is to uninstall if moving to a new major version. In fact, you can do that every time if you like.

Do back up your VM folders (entire .VirtualBox tree), in case you want to go back.

Probably the most important change in v4.x is the move to a self contained folder structure, making it easy and practical to back up, restore and copy VMs. However VBox defaults to backward compatibility, so it will use the existing v3 folder structure until forced to do otherwise. So, one of your first projects should be to upgrade all VMs to V4 layout. The easiest way to do that is to use the VM cloning feature introduced in v4.1.0. The clone will be identical except that it has the v4 layout - though note that some guests may notice changes to disk and VM UUIDs, network MAC addresses etc.

Once you have a set of converted VMs you can delete all of the old v3 ones. Finally delete .VirtualBox\VirtualBox.xml as this forces VirtualBox to create a new (blank) one in the latest format. It also causes your VMs to be no longer registered, so use the Machine|Add... menu item to re-register them. You'll now be bang up to date, optimized for running with v4.x.

Thank you; thank you! A remaining question: any reasons to avoid 4.2.6 in favor of 4.2.4?

No reasons that I know of. 4.2.6 is merely a maintenance release coming after 4.2.4, i.e. it should have additional bugs fixed, no (significant) new features added. There's always someone on the forums complaining that <insert latest version here> is terrible while <insert any previous version here> works great. I wouldn't be put off by that.