The Modular Messaging platform is compatible with the current release of Aura Communication Manager. There are some features that make Aura Messaging more flexible to manage and deploy that are not available on Aura Messaging, so there may be some reasons to look at upgrading.
A question a customer should ask is if there are other features on the Foundation Mobility platform that are needed, and compare that cost to the cost of upgrading the licensing for the Modular Messaging. Customers may find the value additions in Foundation Mobility, and the costs associated with the bundling, make a better argument to replace the Modular Messaging as opposed to upgrading.