Hi 

It seems it's not working this way since 8.0:

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You can also select a work order template only once for each work order.

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Let's say you create a workorder, then you enter some data and save (so not using a template) and you don't change the status (so "assigned").

If you open it again, you can't select a workorder template, the template zone is greyed, even if no template has been used and the status is still "assigned".

So it seems you can only apply a template when you create the workorder, before saving it.