Wish you could get an official answer. From what I can see, there's no easy way to do it. best I can see is to have both open and recreate.
I did try copying the invoice template, and in the top level data area changing the kind to estimate. That made it show up as an estimate according to the editor, but the location field still said invoices. Since templates are actually packages, I went into the package and changed the property list to indicate it was an estimate, and that did make it show up in the estimates menu when you go to create an estimate. Unfortunately it still has a bunch of invoice references so it had lots of error text show up on the preview.
Obviously all the above can't possibly be part of a recommended way to do it, so hopefully someone from Marketcircle will chime in with a supported method or work around, or at least confirmation that it isn't something currently supported or easy.
