Easy mate.
You can cut and joint the new TV plug onto the end of the existing embedded mains lead no problems.
Firstly though, can you confirm the number of holes in the new and old TV mains connectors, 2 or 3?
This is important.
Then it depends whether you want to do it in a small wall box or an in-line joint.
There's various ways to do it, but a good strong in line joint like this
http://www.screwfix.com/p/teetube-ip...le-joint/81353 would be ideal and is and easy to use, strong, safe solution that would satisfy safety regs.
It's waterproof (doesn't need to be of course) but that means it securely clamps the cables so they can't get pulled apart (kids around?)