in a remote reservoir coilover the oil and nitrogen are never mixed as you may know. Think of it like brakes, you don't want any air in the oil side.
The proper way it to discharge the shock, remove the shock shaft, dump the oil out. Replace the hose. Then the floating piston in the resi it set to a specified depth. Then the shock is place shaft up in a vise and filled with oil. Some oil will fill the hose and part of the resi. Then the shaft, seal carrier is reinstalled and stroked slowly to move any air through piston . This allows air to rise to the end of the shock body and not get trapped in the resi or hose.
I do not know what the depth of the floating piston is on a Bilstein resi.
But...
In a pinch you could relieve the nitrogen charge in your resi, hold the shock held shaft down in a vise. With the resi held higher than the shock but still hose side up. Remove hose from resi and set the resi aside hose port up so that no oil is lost from the resi. Then hold the hose above the shock and remove the hose from the shock body trying to lose as little oil as you can. Naturally the oil is the hose will leak out.
Install new hose onto the shock body. Hold the hose above the shock and fill the hose with new oil so there is no air in the hose.
Connect the resi super fast like a ninja and try not to loose and oil (let air in). Recharge the shock with nitrogen and brag to your friends about your new ninja skills, ride out. 
