One common approach to network planning in live IP & MPLS networks is simply to monitor the link usage on the high-capacity links and plan to add more capacity if the usage is > x% (say 60%) at the peak hour.
This means that the network grows without re-optimising or otherwise changing the topology - just adding more capacity on or over existing links. The aim is to investigate when / whether this approach works and when you can do better by adding new links between nodes that are not already connected.
Learn more by checking out David Woods' latest blog on reoptimizing your topology to cope with increased or changing traffic.