You
might be able to do that safely if you used the right barrel. But, I did not see any .40 barrels that
www.Glockmeister.com says will fit a 10mm.
For example, I bought a G19 barrel that I use in my G26. That is the same caliber.
"This is the factory Glock 9mm barrel that comes stock in the Glock model 19. It can also be used to convert the 19C to shoot as a non-compensated. Using it to convert the 23 or 32 to shoot 9mm is not recommended and unsafe as it is not a 100% fit.
This barrel will work extended in the 26."
Glockmeister also sells Glock factory barrels which will convert a .40 G27 to .357:
"This is the factory Glock .357 barrel that comes stock in the Glock model 33.
It can be used to convert the 27 to shoot .357 sig. Using it to convert the 26 to shoot .357 sig is not safe as it will not fit into the slide properly.
Calibers can only be converted down, not up."