Applying Agile Methodologies to Help SaaS Integrations to Be More Predictable

In the realm of software development, agile methodologies have firmly overtaken outdated waterfall techniques for designing, developing, and releasing new software products to the market.  Agile software development consists of a set of principles for which requirements and solutions evolve throughout the course of a product’s life.  For SaaS based software models, however, a large majority of the work that project managers are involved with are not projects involving brand new products. Many projects involve working within the confines of a system that has already been defined and constructed in some manner.  This may include things like data models, business logic, user interfaces, etc.  In addition, the date and cost for the project are often times known and fixed up front, at least for the initial target release.  There is little room for the natural evolution of a product, which agile development suits so well.  So how can agile methodologies be applied to SaaS based project integrations while they have so many inherent constraints upfront?