Please fill out your profile so we know what car you have. I will assume you have a 65 from your handle. Unless you have unusually long heater hoses, it will be necessary to drain the coolant so they can be disconnected. This is required because the blower motor is sandwiched between the heater box and firewall. if you have never had it apart, I would suggest installing a new foam seal kit as well. Not very hard if you have the heater case out anyway. Good Luck!!!
You could use a pair of vice grips to close off the two hoses running to the heater core, then put down plastic and use a bucket to catch what the core bafts out when you disconnect it. Watch out... coolant is yucky stuff.