Thanks for the prompt reply Andrew.
I might have not been very clear. The aim is to generate a schematic and a symbol programitically, instantiating several modules under a new schematic and a symbol which are connected to the above hierarchy level. The instantiation might have different bus widths, so the pins have to change accordingly. I'm not trying to just modify the selected instance, which, I realize may not be ideal, but right now, using a scratch cellview (a copy of a generic symbol-schematic pair and editing the corresponding pin names and widths under that symbol.)
I realize that the approach I'm taking (instantiating the instance first and then trying to edit it) may not be correct. Is there a better way to approach this?