Recently I upgraded from Houdini 10.0.xxx 64 bit to 10.0.465 64 bit (all running on Windows Vista 64).

I had made a model in the previous version and locked a SOP, where prior to this SOP in the network was all the primary geometry in place. I locked it, since it made loading up the scene in Houdini a whole lot faster. The model is fairly large and complex if that matters.

Then I upgraded and opened the hip file back up. However, if I unlock the node, the geometry is broken, about 15 to 20 nodes upstream. More recent versions of Houdini do the same thing, unlock the node, and the SOPs are busted.

Any suggestions on how to fix this? Or am I stuck saving off the locked model as a binary, and then remodeling the busted SOPs from scratch again?
Is this normal for this to happen?

First make a backup of your original .hip file that has your SOP locked.
Now in your scene, unlock it and middle-click on the first upstream SOP that has an error. What's the error message and what type of SOP is it?

The first error node is a polysplit node, the error is :
An error occurred while processing “41912e0:0.686233”
The node just above it is a polybevel node with a warning around it claiming that the group specified is invalid.
The last good node, the next one up, is an edit node.

When you MMB on the polybevel node does the group you are using exist and have any members in it? If not you need to track down where that group was created, there might be another warning or error message somewhere else that relates to that group.

What Simon said. It looks like the problem is that polybevel node which is no longer bevelling, thereby leaving the geometry with a different point count. So then the polysplit is saying that the point number it is trying to process (41912) doesn't exist.