Tesla has always shown it's well-schooled in the law of supply and demand. If the Project Highland refresh increases demand, the Model 3 price will go up.
However, after the Project Highland M3 is officially announced, the demand for leftover pre-refresh M3s will likely decrease. Will it decrease enough to see the return of the elusive 2019 $35K M3? Doubtful, but the refreshed M3 will almost certainly foster reduced prices for used M3s.