It depends on what the next chord is.

"Guide tones" is just another way to say "voice leading" and you need to _move to another chord_ to really say what happens moving tone to tone.

The chord third and seventh are the usual tones when talking about "guide tones", so... 

  - you have the third to handle as any other third as "guide tone"
  - the ostensible seventh of the chord can be treated as any other non-chord tone, for example as a passing tone, neighbor tone, part of an enclosure to a following chord tone, etc.
  - you can decide to modified the harmony on-the-fly, for example is the following chord were `Cm` you could add a minor seventh to a `G` major triad to make `G7`, where, in terms of "guide tones", a seventh resolves to a third
  - you could use another chord tone as your "guide", for example in a progression of `G Cm`, you can use tone `G` as the guide, the root of `G` then becomes the fifth of `Cm`.

> If I decide to just throw in a minor or major 7th, am I changing the function of the chord?

Depends how you mean "function." If you mean the standard idea of _functional harmony_, where functions are pre-dominant, dominant, tonic, then "no" adding a seventh won't change the function. In other words `d(dim) G Cm` and `dmin7b5 G7 Cm6` are both functionally pre-dominant to dominant to tonic regardless of extensions or added tones.