Acts 20:17,28 presbuteros = elders = episkopos = bishops = poimen = pastors = the same people/functions, namely the elders of Ephesus church. Just a point of info.episkopos=bishop
presbyteros=elder
diakoneo=deacon
poimen=pastor/shepherd
How do you figure them all to be interchangeable?
I would venture to characterize our thinking as sometimes a kind of "settling out" of revelation and sometimes an "unfolding". With respect to holy orders it is (we think) as though a number of polities were tried and different orders came and went, but there was a general "settling out" of three ordained orders, bishop, priest, and deacon (like you didn't know that already) with functions that were defined over time.
In the early days things wee not so defined or, comparatively, crisp. But elders as sort of sub-apostles to the successors to the apostles, the over-seers, and deacons as administrative4 assistants with an emphasis on the "social welfare" of Christians seems to have been worked out by, say, the third century, if not before.