How to update the type of a field

Okay. My takeaway from that thread is that there are differences between the default template that I'm using and the template that the person/people in the initial thread I linked.

But which differences matter?

Attempting this (with the obvious change of the index name), wiped the template again.

Is it supposed to be adding it to the existing template? Or should I be copying my existing template, editing this part in, and then posting it?