There is no real way we can answer this question with the information provided.
You say the customer needs to upgrade the network but there must be a reason for that ie. people don't just decide they need a new network (well most don't anyway :-)).
You need a set of requirements from the customer ie. what do they want from the network, what are they trying to do that the current network does not support.
If they feel the current network is not meeting their needs and it is an obstacle to them then you need to find all of this information out and only then can you work out what to do.
If you mean just upgrade the IOS versions to more recent ones that is a different question. For that you would need to find out what features the devices are already using so you need to make sure any new versions continue to support those features , you need to research a stable version and make sure any devices you are upgrading have enough memory for the new IOS.
Either way you can't really do anything until you understand what exactly is needed.
Jon
