As far as I can tell, for at least a couple years now, the ATI installer program removes old drivers and old CCC before installing the new one -- at least, that's what it says during the installation.
I ask for custom installation, but usually just click through. By doing so, I can "opt out" of added non-driver software that might be in the driver package.
During custom installation, the installer tells you what parts of the driver set are being changed and which are not. If you have an ATI chipset motherboard, it may change a motherboard driver -- I let it. Not everything gets changed every month. Things that are installed previously but won't be changed this month are shown by a grayed out check box with a check in it.
Old drivers used to be an issue with both ATI and Nvidia. ATI then provided a "cleaner" program to uninstall old drivers. Now its an automatic function of the installer program (the installer is updated often and loads automatically from the same one GPU driver download -- that is, just download 9.10 for your OS and click on it -- the rest is automatic).
Painless. I do it every month without issues -- including on Windows 7 RC and on my current "retail" Windows 7 HP.
_________________________
ATI Sapphire HD7970 OC - Eyefinity 5760x1200 24", 1xDell-U2410 H-IPS + 2xHP-ZR24w S-IPS, Extended 23" Samsung cPVA, Ceton InfiniTV 4, Bulldozer FX8150@4.5GHz w/Swiftech Water Cooling, 16GB GSKILL PC3 @1866, ASUS Sabertooth 990FX, Corsair 120GB SSD, WDigital + 3x Seagate + Hitachi + 2x WD Ext = 10.0TB, Sony DVD, OCZ ZX 850W PSU, CoolerMaster HAF922, TM Warthog HOTAS, TM T-Flight Stick X, TM Cougar+FSSB & CH Pedals, Saitek X52 Pro & Pro Combat Pedals, TrackIR5 w/TC Pro, Windows 7 HP 64b