In my experience, changing to a slower burning powder with a longer barrel is not a mandatory consideration. It's an option that could be considered to refine a load. Of more importance is the effect of the new barrel in terms of accuracy, velocity, and ES. Using your current load in the new 31" barrel will likely produce a higher velocity, and require a change in your ballistic information. I have more often found that switching from a long barrel to a short one may require changing to a faster powder because of the blast produced by not having enough barrel length to burn all the powder. For me, the best situation when changing to a different length barrel is when I get the same, or better accuracy and ES, and only have to modify my ballistic information any change in velocity. This all assumes the previous barrel was a good performer and being changed because I burned it out. If the barrel was changed because of poor performance, I would consider a slower burning powder in my load development with a replacement barrel that had the length to burn it.
