You'll need to drain the front brakes in order to prevent it from spilling when you remove the calipers. Then you can simply use the brake bleeder rubber plugs (on either front or rear brakes) for plugging the open ends of the lines. If you're missing yours, these can be found at most auto stores (Napa etc).
Brake fluid absorbs moisture, so I would recommend you completely drain and refill the entire system, in order to remove any moisture that may have entered through the front brake lines.
Where are you having your calipers rebuilt? Are you planning to powder coat them afterwards? May I ask what you've been quoted for the rebuild? (I'd like to have mine done as well)
