Can't do the reverse, the conversion from 40S&W to 9mm is possible because you're going to a smaller caliber, so the conversion barrel has the Outside Diameter of 40S&W and Internal Diameter of 9mm (will have very thick barrel walls). Going to a larger caliber does not work, the walls of the barrel would have to be too thin (if even possible) and would not be able to handle the pressure.
For cheap I say get the G26 and look for a G27. I think two guns are better than one with conversion barrel. I did have a conversion barrel for my G35 (40S&W to 9mm), had to get 9mm mags and a lighter recoil spring to reliable work with 9mm. Could also have used a different ejector as the one for 40S&W is positioned differently. It was just for cheaper practice, but if I was thinking of using it for personal defense it would not be reliable enough for that.