Your concerns are legitimate, but ultimately, what matters is content. So go on and edit at will.
There are some devices that one can use to accommodate to a degree such concerns:
Using a section labelled "ADDENDUM" to add something at the end of the post, if the new material is not a revision of the current version, but an extension.
If references are to be added, one could enclose them in an "UPDATE - END of UPDATE" demarcation. Eg. "The Law of Demand has been empirically verified (UPDATE: see ref 1, ref 2, END of UPDATE).
If major reworking is what happens, then a note at the beginning would not be out of place:
E.g., "Update XX/XX/2015. This post has been extensively reworked to address some issues raised in the comments". (or "because I realized there is better way to structure the argument").
And other things to that effect.
In my opinion it is good practice to use such devices, not only because it tries to maintain rapport with the past of the posted answer, but also because it shows to future readers that people here care about the feedback they receive and about their posts and go back and improve them.