Why would you change the template?
In attached()
after super.attached()
the element is already rendered.
You can access the template with this.templateInstance
and try to change it before super.attached()
(or in the constructor or in polymerCreated).
I haven't done this myself yet.
It's no problem to change the rendered element.
If you want to avoid that the this change is visible, you can set a (CSS) class initially which is defined to render the element invisible and after you are done with your changes, remove this class.