This is a mobile client for RuneScape - not a separate version of the game. The interfaces will be optimised for mobile, and there'll be persistent cross-platform play with the desktop version.
That means you'll log in with your existing account, continuing the adventures of the character you've spent so much time levelling. And of course, you can hop back onto the desktop client whenever you want.
Awesome! When can I play?
Old School will hit mobile devices this winter, with RuneScape hot on its heels in 2018.