There are many things to consider when migrating a legacy IT estate to the cloud. The first though must be what are the motivations and expected benefits. Many organisations have many decades of developed software running on private infrastructure and migration to the cloud is something they think they should do.
Migrating an estate to the cloud incurs a significant cost hurdle as new functions are required just to support migration activities. Often the benefit is minimal as only limited efficiencies can be found from closing (or worse partial closing) of legacy applications and data centres.
What is needed is a target systems architecture aligned to business benefits and vertical product supporting IT Stacks.
The systems architecture should reflect management of intermediary states between internal hosting and public cloud. The management of intermediary estates can easily increase an organizations run cost; for example if Corporation A decides to migrate all of its channels’ IT to a public cloud it will need to build an integration from public to private infrastructure, lease connection between new and old sites, provide a security wrap and identity mgmt function across internal and external clouds and finally support the operations for managing these new systems.
The benefits to support all of these new cloud enablement functions will be high. This does not mean it should never be done but the business must address how benefits like improved time to market will be substantively realised.
A TOGAF business architecture should be included before migrating as migration for the sake of hosting will only ever be a platform change. The balance has to be on how much change your organisation can stomach in a single move. Always consider that the SaaS services you are considering will probably be more configurable than your legacy estate. So don’t fall into the myth of business architecture as business change does not always have to be front loaded.