I found that if I made changes to a workflow and published a new version (even without changing any input/output params), the BP that calls the workflow will still call the old version of the workflow until you click this button in the BP workflow activity and re-publish the BP.

This is very unexpected. I would think that if a new version of the workflow was published, that would be the current version that is used by anything calling the workflow and previous versions are only for backup purposes.