There is a regression in 6.x in OnChangeAjaxBehavior.
Now I noticed that in 1.5.x and earlier it actually behaves like simple AjaxFormComponentUpdatingBehavior("onchange") for non-text form components.
I see two solutions:
1) change the event to "change.inputchanged". Need to verify it but I think it will be fired when used with DropDownChoice
2) rename it to OnInputAjaxBehavior and update its javadoc that it applies only to text components. It can even throw exceptions if used with something else. Just as AjaxFormChoiceComponentBehavior does. And explain that AjaxFormComponentUpdatingBehavior("onchange") should be used for non-text form components.
I'm more in favour of 2).