Adding something to a loop?

What to add? Coolant or additive, no of course not however a new block? Sure you'll need more tubing anyway, no?

Safest would be to drain and reconfigure.. There have been some posts recently about clamping while they fit a new block in place of the existing one but that depends on how lucky you feel.

Honestly, unless your running some special coolant that's tons of money the drain and replace is the best IMHO.

I added a waterblock to my gtx 280 this week without draining... of course I had a pair of needlenose visegrips which made it easy

I would drain and refill. Its not hard and not time consuming.

I've been through this recently... unless you have VERY strong clamps, just drain =) I bought clamps and they still didn't do it.

If you do have uber clamps that can pinch the tubing, be sure to use some cloth to prevent damaging it, teethmarks = weak spots.