Wow - that's an expensive way to go... not that I don't like us making money, but depending on your needs, you might want to find a cheaper way to go (do you just need off-site storage, or do you need it to be accessible on the web by other people too?)
A dedicated machine would be a much better deal. As far as the moves go, we generally take care of the moves when you're signing up for a dedicated plan, though I'm not sure if there would be a fee involved in moving 3 separate accounts). As far as the charges themselves, never hurts to write into sales / support and see - maybe they'll hook you up with a better deal on a dedicated plan. It's also possible that we'd be able to work out some sort of special deal for your disk space (though we don't normally do that anymore).
I wouldn't assume that. :>
The man pages have some examples that might be helpful, or I'm sure people here could offer some examples. Rsync works really well - the main thing to watch out for is that including / not including the trailing slash on directories can make a big difference.
Something like this would work:
rsync -auv ~/pictures.example.com/photos/ email@example.com:example.net/photos/
(transfer the entire contents (recursively) of ~pictures.example.com/photos/ on the local machine to a remote machine
This assumes you're on the machine for domain 1 (example.com in the example) and that you're transferring them to the machine for domain 2 (example.net), and that "user" is your username on the machine for example.net. It also assumes that your domains are in a directory with the same name - this makes the example more confusing to read, but it's the normal convention on our system.
That make sense?