Scheduled swiching between DNSs

[adopilot]

Idea is flowing:
We need to block social sites at our company inside our LAN (a.k. Facebook, Geni, and many others)
But I want also to left these sites open before work time and during the pause.

For first Task I thought to use OpenDNS.com as DNS server which allowing me to block these sites.
And during the launch break I want to dynamically change  my DNS to local ISP DNS which allowing visiting those sites.

What will be proper way to do this:

My idea is to do port forwarding for DNS calls inside Lan to Wan according to time scheduled.

Or Ill try make task inside Pfsense OS which is going to change DNS server for connection.

Or is there better way accomplish this task, Using some others methods

Thanks in advanced
Admir