You can request your existing DPS professional account provisioned for DPS 2015 from this link : https://adobe.allegiancetech.com/cgi-bin/qwebcorporate.dll?CM2KP9. Once you fill the request form, Please allow for up to five business days for your account to be set up.
Both systems are completely independent. Asking for access to the new Digital Publishing Solution does not change your access to the Digital Publishing Suite system in any way.
If a client with 2 Professional and 2 Enterprise app in the Store (planning to migrate to DPS2015 in the next 6 month) request the account setup can still use DPS2014 account management?