Migrating Master/Media server

You did not mention your current NBU version?

W2012 is only supported as from NBU 7.6.0.1 and W2012 R2 as from 7.6.0.3.
Symantec does not commit to a 'preferred' OS.

2GB Ram is not nearly enough for a master/media server. 
Recommended minimum memory for NBU 7.6 master is 8GB (as per NBU Installation Guide).
Master server should have a minimum of 2 physical cpu's.

Configuration and specs of master and/media servers depends on the size of your environment - the amount of clients, data size on clients, amount of backup jobs per day.

Some reading matter to help you make decisions:

NetBackup 7.6 Best Practices: Optimizing Performance

Updated NetBackup Backup Planning and Performance Tuning Guide for Release 7.5 and Release 7.6

Using catalog backup and recovery to transfer NetBackup catalogs between Windows master servers as part of a hardware refresh
http://www.symantec.com/docs/TECH77447

Statement of Support for NetBackup 7.x in a Virtual Environment:   http://www.symantec.com/docs/TECH127089  

NetBackup 7 Operating System CL : http://www.symantec.com/docs/TECH76648

Master Compatibility List: http://www.symantec.com/docs/TECH59978 

Currently we are using Symantec NBU 7.5.0.6, which runs under Windows Server 2008 R2. For the client machines, do I need to update them also?

Recommended to update but they will work with 7.5.0.6 on them.

You will need to upgrade to 7.6.0.x on existing master, then use TECH77447 to migrate to W2012.

As stated above, 7.5 clients will work with 7.6 servers. See Version Compatibility TN:  http://www.symantec.com/docs/TECH29677 
Best to upgrade clients to get full benefit of new features, enhancements, bug fixes, etc.

Can we have both a master server and a media server (which both runs under Windows Server2012 R2) in a Windows Server 2012 R2 Hyper V? Is it recommended? 

Disk media server is supported in a virtual environment, not tape.

Please go through all the links that I have posted yesterday.
Especially this one:

Statement of Support for NetBackup 7.x in a Virtual Environment:  http://www.symantec.com/docs/TECH127089   

My recommendation is to keep media servers as physical.

Don't confuse yourself between Hyper-V Server 2012 R2 and Windows server 2012 R2 with Hyper-V role.

If you have the Windows server 2012 R2 and if you are just installing the hypervisor role on it, it will become windows server 2012 r2 hyper-v. This is supported by NBU as Master/Media/Client and shows in SCL

If you are having hyper-v server 2012 r2 - it is not listed in SCL as a NetBackup Master/Media/Client. Just an FYI I have in my environment hyper-v server 2012 r2 and backing up as client without any issue. But never tried master/media on hyper-v

Actually it is Windows Server 2012 Datacenter as host machine OS and the client (Master Server) will be on hyper v with windows server 2012 R2 as its OS.

I have another issue. Currentlly our master and media server resides in the same server. Then later we'll migrate the master to hyper v while media server will be running under windows server 2012 (it will become 2 media server now)

Not sure what the 'issue' is with this?

Seems you will have dedicated master and 1 physical media server? Not 2 media servers. Unless virtual master will be disk media server.
Best to use disk for migration process in TECH77447.

You will need to remove all tape (Media Manager) devices and STU from master server before the migration.

Add physical media server, add devices and STU and update policies.
Move ownership of all assigned media to media server with 'bpimage' command.

Please read through the 'General guidelines for support' , 'Virtual master server' and 'Virtual media server' sections followed by Table 3 in TECH127089.

Not sure what else to say about your intended migration....
I think the answers already provided here plus all the documentation should be enough?