How can the minimum Proof of Work difficulty be changed on a live block chain?

[mammix2]

Wondered if any one can help with this.
Is there a way to change the minimum Proof of Work allowed on a live block chain?

The minimum value is controlled here with this declaration

Code:

CBigNum bnProofOfWorkLimit(~uint256(0) >> 20);

I want to change it to something like this and make it take effect on a future condition.

Code:

CBigNum bnProofOfWorkLimit(~uint256(0) >> 22);

I can change the actual value within a function and initialise it to set the different value based on a future condition such as nTime or block height. But how could i make this take effect, since the minimum proof-of-work difficulty is already stored in the genesis block and the current difficulty adjusts on every block thereafter.
Is there a way to never allow the minimum proof-of-work difficulty to drop below the new limit?