You may still need the radio code for an older A2 - don't assume you don't.
If you've got some jump leads, you could always rig up a bridge to the new battery before disconnecting the old one, so there's no loss of power.
Clamp the jump leads to the loosened terminals on the car cabling and then to the new battery, which you can put in the boot next to the old one and to one side. Then when you've removed the car cables from the old battery and moved the old one out, you can lift the new one in and pop the terminals back on. It's easier with an assistant, but doable on your own.