Crew Changes?

I was wondering today, do any lines have a crew change in the middle of the line? I would imagine it would be a very long run on the A or the F train and it would make sense to have crews focus on tracks only on 1 half of the line.

unless all hell breaks loose, you're completing your run from terminal to terminal.

First of all, if what you suggested were true, there would be massive delay associated with the change over point.

Secondly it would be a waste of money to provide two crews per train, per trip in each direction.

The A is by no means the longest subway line in the world. there are no mid line relief crews on London's central line, and that's longer.

did they not have crew changes many years ago (in the days of the double fare) at Euclid Av for the Rockaway trains?

That was because the Rockaway line was a separate division of the subway when it first opened. It was not actually a part of the IND. The crews were Ex-LIRR personnel hired to operate the trains.