So, I made sure to follow all the best practices right from the beginning... use content types, create SPD reusable workflow, export the workflow as a template (sandbox solution WSP)...
Built it out in DEV environment, and was able to deploy into TEST and then PROD with no issue.
Everything there is good.
Now it's time to update the workflow... back in DEV environment, I've made the changes in SPD... tested the changes in DEV, all is good... I've again exported the workflow as a template...
but when I import the new WSP back into the solution gallery in TEST (I've tried using the same name as before, or using a new name) and activate, nothing happens.
None of the workflow files (XOML, XOML.Rules, WFConfig) have been modified (timestamp wasn't changed when the WSP was activated)... none of the changes are present in the files... the new task field wasn't added to the site columns... and when I start the workflow from the list, I don't see my changes (in the task form, in the VDW, etc).
so...
What am I missing?