From the TechNet here are the Skype4B pre reqs links you need
- Windows Firewall Service must be running
- At least 32Gb of available disk space !
Latest Lync 2013 Server CU (Feb 2015 currently)
PowerShell RTM version 6.2.9200 or later
SQL 2012 Express requires at least SP1 but SP2 is available
I would really think if your using 2008 R2 OS life time and improvement to Windows Fabric requires Server 2012.
Recommend OS for Skype4B Server 2015 is 2012 R2.
To perform an InPlace upgrade you will need a workstation / Server they has no Lync admin / tools installed 🙂