There is no upgrade path, iWork '09 is a completely new purchase. You can actually have both iWork '08 and '09 running on the same machine.
iWork '09 is well worth the very cheap price and you can tell your employers that installing it with the same Apple I.D. from the Mac App Store means they can install the apps on all the Macs in the office at no extra charge. Which makes it even more of a bargain.
To use the best tool available for the job, especially when it can be bought with petty cash, is a no brainer,