The catalog file will be copied and the copy will be converted to Lightroom CC 2015 format. So, yes, you will need the space on your external SSD. This new catalog file can be opened on any computer that has Lightroom CC 2015 installed.
The photos are not touched, so they will take up the same amount of space as before.
thank you for the quick reply.
one last question. I'm assuming during this process i will be asked where to save the new catalog? My catalog is about 600gb so i'm juggling space to copy/save/move, etc..
Sorry, I tried to make this clear, but apparently not.
Your catalog file is what will be converted, this may be a few gigabytes at most. No, you do not get asked where to save the new catalog.
Your photos (which make up the bulk of the 600gb and are not technically part of the catalog) will not change or be copied/duplicated, you will not need another 600gb, you will not need even a single extra byte for your photos.
Ok, got it. So it basically 'converts' the catalog in place. I have it backed up should anything go wrong. So this seems straight forward.