Will changing to factory 18's require a speedometer calibration?

They are factory 18's I believe with blizzaks installed. 235/50R/18

Stock 17's are 215/65-17. I don't plan on putting the 17's back on, and going to 20's next spring.

 

Your Speedo is basically wrong from the factory (if possible check it with GPS), it more than likely uses the same calibration for both sizes from the factory.
With that being said your speedo is about 2 1/4 MPH off at 80 so I wouldn't worry about it if you are not using a tuner for something else.

 

IF your speedo was reading accurately with the 17's, and IF the outside diameter of your 18's comes VERY close to the O.D. of the 17's, your reading should be reasonably close... same holds true for the O.D. of the 20's you propose to buy. Sidewall height is the big variant between the sizes; outside diameters can be fairly similar, but pay attention to this when you are shopping.

 

https://tiresize.com/speedometer-calibration/