It used to have an upload folder on your PC. You copy photos into it and they flow to your devices. I'm not seeing it now...but I have not checked recently to see if that functionality is still there.
11/18/13 Edit. I was wondering if this functionality I mention above still existed as some parts of iCloud seem to have been updated over the past several months. But it seems it is still there, at least on my office PC running Win7. If you go into the My Pictures folder, then into the Photostream folder...there you will see three folders...My Photo Stream, Shared, and Uploads. In the My Photo Stream any image you take on an iDevice will land here in full resolution (remember, between devices, you don't get full resolution for some reason). The Shared folder is for sharing I imagine, but I have not used that. The third folder, named "Uploads", is there for moving photos out to your devices from your PC (I suppose on MAC too, but I have not checked). I use it for the purpose I have described. I don't know what the resolution of the transferred.
So, I don't think if you have old photos on one iDevice that they all get sync'ed once you turn on iCloud, but I honestly cannot remember if this happens or not. A sure way to get them where you want them is to take them from the photostream folder and copy them into the upload folder.
Photostream has always impressed me as a little nutty in operation. I mean, what goes where is not always clear. I know if I take a photo on my phone, those will show up on my other devices in the photostream folder/album. Likewise for my iPads. If you try to delete something in a photostream folder on an iDevice, you get a message saying that this photo will be deleted from photostream on all devices. But I don't think that removes that same photo from the original device's camera roll, just from its photostream. So as far as photostream is concerned, the original photo stays on the original device until you delete is from the camera roll.
As for retro operation...I really don't feel confident to answer as I can't start over. I guess one needs to look in the manual or go to the Apple website to see how this is supposed to work.
One more thing. It seems that Photostream is actually paying attention to the 1000 images shared thing that it was first said to have. For the longest time, at least on my systems, that was ignore. Or, perhaps because I have three iDevices I get 3 times 1000 images stored on my PC. Right now the folder says I have 3023 items on my PC. My 3nd Gen iPad has 970 images on the My Photostream album. It's the same on my iPad mini. I have 971 images on my 4S My Photo Stream album.
I'm sure this is all clear as mud.