I think that it should hack the sentry/building for your team, BUT it should work like the red tape recorder in a way:

It would be 2x or 2.5x as fast as the red tape recorder, but when the building is about to explode from being sapped, it would change color and build back to the level of building it was when sapped.

This has one downside that makes it realistic in the game: when the sapper is removed, if it is placed back AFTER downgrading a building and proceeds to fully hack the building, for example lets say a Bi-Decoder is placed on a RED lvl3 sentry. The RED Engie owning the sentry hits off the sapper when it hits lvl2, spy kills engie, places sapper back. This would end up fully hacking the sentry to BLU but it would be a LVL2 sentry, not a lvl 3.