Different tire size front and back?

[J.Belknap]

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
275/40-20 4.3in 14.3in 28.7in 90.0in 704 0.0%
315/35-20 4.3in 14.3in 28.7in 90.1in 703 0.1%

I'd worry about changing over to a different brand and not changing all four, as sizing varies slightly between models, and brands.

Interesting that, as your rears wear down faster than the fronts, they become closer to a perfect diameter match, front to rear.

You could run slightly lower pressure in your new tires. I'd do it with no regrets if replacing with the same brand and model.