You don't have to clamp the coolant lines, but you will have a little coolant loss if you don't. Don't disconnect them and take a coffee break. You can actually do one at a time. Depending on the conditions of you hoses(rubber is flexible) . Disconnect one at a time and attach to new one. Slide out the old one and slide the new one into the rubber holder. If you really don't have th confidence to do it, then let your indie do it.

__________________
1995 E420 152k
1991 300D 2.5 348116 daily driver
1992 300E 2.6 196k-work in progress(head gasket)
2002 BMW 745Li 93k