Horse Rebus #695

If you're new to Rebuses, you can read how to solve a Rebus here (link opens in a new window).

Do you need a little help? Click to reveal a clue to the above Rebus.
Here is a clue to help decipher the hidden meaning. To make major changes in an activity that has already begun, to choose someone or something else after it is too late.
Do you think you have got it? Click to view the answer.
The word “Hoarse” has been crossed out and been changed to “Horse” midway between the word “Stream”, to give the answer: Change Horses In Midstream.