I’ve made a custom attribute which is attached to all my texture maps in a lot of my models. This attribute has a few radio buttons and some other widgets. A few days ago I renamed some radio buttons and removed one of the labels on one radio button so now the attribute has a different definition.

Now when I load old art models which had the previous attribute attached, I get a bunch of errors about an index being out of range. I tracked the problem down to the change I made with the radio button. The previous radio button group had three options while the new radio button group has only two. The error happens when an older model had specified the third option in the old attribute, but since this option is no longer in the new definition Max will display the error.

My question is about how to make updates to attributes without breaking older ones saved with a different attribute?

Is this what you need:
custAttributes.redefine <def> <definition_string>

Max 9 through 2009, XP-Pro x64 SP2
ASUS EAH3450 Series (Driver 8.470).
Core 2 Duo E8400 3GHz, 4Gb Ram, DX9.0c.

I think redefining it would work.  I’m a little worried about previous settings not correctly mapping to the new ones.  For example, if the third option was selected on a radio button for the old definition what option is selected on the new definition which only has two options.  Do I have control over this or does redefining automatically reset everything to the default? Resetting to the default would mean doing a lot of re-work.

Yes, redefining will reset radio button if removed option is selected. When you dont need anymore the third option, you can test whats happen if first set option 1 or 2 and then redefining attributes. Just try merge on empty scene for safety.

Max 9 through 2009, XP-Pro x64 SP2
ASUS EAH3450 Series (Driver 8.470).
Core 2 Duo E8400 3GHz, 4Gb Ram, DX9.0c.