To switch the template type between a Workflow/Timeline, you'll need to clone the data from the original template & create a new version with the desired template type (it may sound technical, but it's pretty simple!).
Note: There is currently no direct way to switch between the template types.
Steps:
Navigate to the folder where the template was originally created (e.g either in the Home Folder/Team Folder)
Click on the Template Library tab
Note: If the template was created in a Team Folder, click Templates tab
Select the template checkbox
Click Edit Template
In the bottom-left, click Show Raw Code
Select all of the text, right-click and click 'Copy' (Note: It is important to copy all of the text, and do not change any of it. If you do, it won't work)
Click the 'X' in the top-right, to close the form builder
Staying within the Templates section, click Add Template (Note: you might need to un-select the original template to see this field)
Click on Build Your Own, enter the details of your new template & select Next
In the form builder, click Show Raw Code
Remove any text that's already in there & paste the code you copied
Click Hide Raw Code to see all of the fields
Click Save when you're done