1. Probably better to replace with 28 mm copper.
2. You will need to drain, or very securely "bung" the cold water storage cistern.
3. You will need to drain the hot water cylinder:
3a. There should be a drain off point, normally where the cold feed pipe goes into the hot water cylinder.
3b. If there is no drain off point, or it doesn't work (quite likely), then undo the top hot water connection and syphon the contents out of the cylinder from there.