Major,
You'd might be interested in reading these two threads -
The Physics of Freezing and Freeze Protection
Winterizing Warning!
The short answer is this - given your location and typical winter weather conditions, you're at very low risk for water freezing in your pipes. The first of those threads basically shows that it would take extended periods of time at below freezing temperatures to be of any concern for stagnant water in standard PVC pool piping. In the 2nd thread,
chem geek basically shows that any amount of water movement in a pipe will make it nearly impossible for water freeze unless the temperatures are far, far below freezing -
Given usual rather fast water flow rates through pipes and most especially if the pool water was not near freezing in temperature, it would take an extraordinarily low temperature to cause freezing of moving water in the pipe.
So even on theoretical low speed pump setting, your plumbing would be fully protected. As a personal example, in the winter my pool water will get down into the low 40's (~38-42F). When the air temperature drops to near freezing (~34F), my automation system turns on my variable speed pump. I have it setup so that it turns on a separate circuit with an extremely low speed set (~1100rpm on the pump and ~200W of electrical power draw). Given the head loss in my system, that speed setting causes barely a trickle to flow from my returns (you can just barely feel the water coming out of the pipe). My pipes are fine and I have never had any freezing issues.
In case it wasn't clear, I was not implying that you
had to switch out your pump motor, only that it would be a good idea to do so given the fact that SWG pools run their pumps as much as 2X more than manually chlorinated pools. 2-speed motors (you'd only swap the motor, not the wet end) are very cheap (less than the cost of a new SWG) and, if redoing some electrical work at the equipment pad isn't too costly, a 2-speed motor is a great way to save on electric utility costs. Obviously up to you based on upgrade cost, but we've had a number of threads here on TFP where folks install an SWG and then they're perplexed (and upset) at why their electric bills are so high...most simply do not realize the added cost associated with having to run the pump with the SWG.