That is quite an oversight! The conservative reading would be that you can create "an object, creature, or force, as visualized by the character.". One such thing per casting. So you create "an orc" and you can make it run around and do whatever you want within the area of effect and that's it. IT cannot change, or disappear, any more than a real orc could.
A very liberal interpretation would be that the spell gives the illusionist complete control over the area of the spell. Anything they can imagine (while maintaining concentration), they can make appear. So you could start with an orc, have it change into a demon, then have a dozen skeletons crawl up from underground, then rain down some fire and illusory smoke etc. etc.. As there is no pratical limit to the duration (concentration), one casting of Silent Image becomes rather powerful. You could cast it when you went on guard duty, or while you walked at the center of the party, and then use it as the situation required.
I guess I would aim for a middle-road. You can create one creature, object, or force (and just one, not a horde of em); but you can then modify that however you want without changing it's type. So you could create a single orc and have it grow wings, or turn into another creature; but not an object or force (that would require another casting of the spell). If even this got out of hand, I'd consider putting in a size restriction (maybe one step up or down from the original category).
The spell description should really say though.
A couple random things to keep in mind: a figment cannot make something real appear differently than it is. And any incontrovertible proof automatically disbelieves an illusion.
