Little league..special pinch runner question

[4x4tx]

Could you please help clear something up for us. In our game last night we had a situation come up for our team and we couldn't agree on the correct course of action. Here is the scenario on what happened...I will use the names Mike and James for the 2 players...Mike started the game and James was the substitute that was "married" to him in the game...Both Mike and James had already met the required playing time in the game. At the time of the game when the question arose, Mike, the starter, was on the field playing defense. The inning ended and Mike came in to bat. He was not due up until 5th that inning. Another player reached base and we wanted Mike, who had just been in the line up on defense to be a special pinch runner for that player. Could we have put Mike in to run, or do we have to wait for his time at bat to swap him and his sub James out of the line up. Like I stated, all mandatory play time had already been met..It was my understanding that you could freely sub these players back and forth?