All accounting issues aside, in the end I think the mass upgrade is more disruptive to business on all sides than the trickle replacement.
For those of you that have done mass upgrades, consider the process.
You select a particular model and config based on that days standards.
You have them all shipped and have a pile of PCs sitting somewhere.
They have to all be unboxed. How many fit on your bench.
You spend time (weeks?) working up your image.
You start to push your image as quick as your hardware will allow.
You start swapping out user machines as quickly as you can.
User questions start rolling in - you're still trying to move computers off your bench.
You realize you have to tweak your image.
You redo the image and reimage the machines that were already done.
Trickle replacement
A few machines show up each month.
You unbox them in your office and put them all on your workbench.
You deploy your image and keep working on other tasks.
You deploy them to a few users and troubleshoot any issues.
Make a note of issues and tweak image for the next round.
Order up the next round and repeat next month.
As new OSs or software comes out, you can try it on your next cycle without disrupting the entire company. You don't have to update everything all at once so that users are getting a new OS and new software. In the mass upgrades, how long do the machines sit on the bench depreciating before they are put in front of a user? As employees are added ordering another computer for a new user can be done quickly. If you are in the 3 year of a mass upgrade, you have to price shop and all that since your original model likely won't be available.