I have explained how proconsulships first came about in this post (under my previous handle). In the Late Republic though the proconsulships were changed from ad-hoc occurrences to something more structured. It is that change which I was referring to in the post you linked in your question. I do hope someone like /u/xenophontheathenian or /u/astrogator will have the time to explain how and what changed (as I said, I know the basics but it is outside of my area of expertise).