90% of your questions really depend on what YOU want!
'when?' is whenever you want her trimmed; for some people they want to keep the full puppy fuzz, others are happy to get it trimmed back when young.
'what length' depends on what you want the length to be! I hate when customers tell me to just do what looks good, cos what looks good to ME might not be the same for THEM! lol. I always get an actual idea of the length they want, because invariably 'short' never actually means SHORT. lol!
'pics?' there are lots of pics on here already, but it depends on what you're wanting too...
'will her coat change?' it won't change, no, because it wont' change what is already there, but it WILL look different, because shorter is generally plusher, and the newly cut tips of the hair will be flat and flush and in general will look denser and thicker simply because the tips of her hair at the moment are invariably broken and 'wispy'. It won't change her coat at all, but it will probably make it appear different.
'clipped or scissored?' depends on the length you want. If you want an inch or less anywhere then it'll probably be clippered with a snap on comb attachment simply because it is sooooooooooooooo much easier than fully hand scissoring (note that scissoring is still required with snap on combs to tidy it all up & finish it) if you want it LONGER than an inch anywhere (usually the legs) then it'll probably be hand scissored. Also taken into account for the answer is how much you're willing to pay; a fully hand scissored dog takes a lot longer and a lot more skill, and will invariably cost a lot more than one I can use clippers on. Let alone trying to get a puppy to stay still for a long hand scissoring session... lol!
Hmm, I don't think I helped much. hahahaha!!
I suggest you search this forum and the internet for a poodles first groom and see what you like and don't like so YOU know what you want rather than have us tell you. lol!