KNoel:

You can always open your spec & make a copy of it as a different filename (also, changing the Specification Options title).

Then you can modify the line spacing, and not be spooked about messing up your forms.

As well, once you have it working, you could have two (or more) specs, named: Invoice-Single Space and Invoice-Double Space, or whatever you decide makes sense. It then becomes relatively easy to set one of your specs as the default, and when necessary, you can switch to the other spec.