Hello Stephen,
Your old Tasks should be able to use the new Workflow as long as the Workflow is assigned to the project they reside in.
Chances are that the old Task Workflow is still assigned to one or more of your projects. Identify a particular project in which the Tasks seem to only use your old Workflow, and edit that project. In the Project settings, you'll see a section labeled "Workflow Settings". Make sure that under the Task item type you have your new Workflow selected, and that the old one isn't being inherited from the parent Project.
Let me know if you need any further assistance.
Thank you,
Tom Harder
Axosoft Support
support@axosoft.com1.800.653.0024 option 3--'Fear the Bug' Podcast--