Why would you do that, it is more expensive (extra profibus CP's needed) and requires a lot of work that you simply don't have with the redundant CPU's. Also any connection with WinCC you have to create fro each CPU (with each tag existing twice, also a lot more expensive) and there is no automatic switch from one to the other. With redundant PLC you connect trough an S7-redundant connection (requires Simatic HARDNET CP's and REDCONNECT option).,
The software redundancy was originally developed for low cost S7-300 controllers, I was not aware that anybody would ever want to use that on a 400 series PLC as there a re far better options available there.
The 400H CPU's are in the same price range as the standard CPU's and they work redundantly out of the box. Furthermore as I have already stated Profinet in a redundant ring configuration is only supported as of firmware version 6, currently only available for 400H CPU's. The setup of this type of redundancy is simple, just go from your CPU to the first station, then from there to the next stations ... and at the final station go to the redundant CPU. This is completely different from profibus where you would have two IM153's for each redundant ET200M rack, one linked to the profibus of each CPU.
So if you want to press ahead with your current plan, you should consider getting the profibus redundancy bundles (ET200M with 2 IM153HF) in stead of profinet IM151. This also means replacing all ET200S with ET200M racks which may be a lot more expensive than buying new CPU's if the IO is already installed.
Hope this helps you, but I would also advice you to get a local siemens representative to help you decide on the best way forward.
Kind regards and good luck with your project,