one example is trid phantasm, can you create an illusion, say you decide to make it look like you are wearing a towel getting out of the shower, then altering to be fully clothed... then continuing to sustain the spell you go get dressed and want to then alter the illusion... you're all out of coffee so you make the milk taste like warm coffee...
how much is too much?
when you cast the spell originally, you created a set of objects, that's all 5 senses, and the function of motor function alters the image created, forcing movement in the illusion...
can you change the fashion of the clothes?
and you change the color?
can you make it look like someone else is wearing them?
can you make that person look like you?
can you make the hot coffee he's drinking taste cold and bitter?

all without recasting the spell... at what point is it no longer the original illusion and you have to cast it again?

this is not intended to be limited to trid phantasm, all sustainable illusion spells are here and available within the context of the query, phantasm simply being the most open to interpretation.
if you have a common sense answer please note it as such, separating it from a RAW supported answer.

Illusions are ind-tricks and in case of physical illusions mind-and-sensor-tricks. Just as you can move the illusion around you can change it. As long as you continue to sustain the spell, you can do whatever you want with the projected image.
Doing this however also has a possible downside. You will not get more hits than the initial casting.

if you can change/alter the illusion after casting then you practically control all elements of the area right? for Trid Phantasm you are "God" of the limited space, able to do anything, make the opponent believe his gun is not in his hand, or that his wife (combined with a mindprobe or other similar spell) is standing in front of the gun... or that all of his attacks are bouncing off, having no effect... I would probably say that the condition of the initial casting is the limiter of the sustained spell. Masking for example is limited already so can be altered within the defined parameters of the spell but for entertainment or phantasm I would think that there is an unlimited potential for abuse if it is not limited to the creation of the casting. however, I also cannot think of a way to limit it as the illusion is complete.

you also don't get any less, Sustained focus level 6, cast the spell spend edge, and bammo...

And anyone who sees through any part of it sees through the whole thing. Movie companies use it because cameras record the physical version of the spell (which is then part of the data -- there's nothing to see through, since the movie is what it is). If you were doing all kinds of sensory manipulation on different people, I'd give them dice rolls on all that and once their brain catches on that this thing isn't real, since the whole spell hangs together... a chain is only as strong as its weakest link.