Getting old photos from Camera Roll to Photo Stream
I'm doing this by using Photos on a Mac. I connect the phone to the computer and Photos lets me import and then delete the originals. 

Once that is done, the Photos app pushes them up to the iCloud Photo Library and the phone has free space to re-download the photos over time.

If you have a PC or don't want to use Photos, you can transfer the photos using iTunes and then use a web browser to upload the photos to iCloud.

Having a computer to do the upload is working much better than having an iOS device which goes to sleep and uses battery instead of wall power.

I've gotten 15k photos into the cloud so far with 10.10.3 and it's been solid for a day. Fingers crossed it stays that way.

Also, as Buscar웃 astutely pointed out - your iCloud Photo Library can grow to arbitrary size but the Photo Stream has limits - 10,000 photos per 24 hour upload period / 25k per month. These limits are only in effect if you don't store all your photos in the iCloud Photo Library using iOS 8.3 or later. Then, you are limited by your total storage space and bandwidth as far as I can tell so soon after release of the product.