Maintenance takes up about 50% of all IT budget [1]. Most individual pieces of software will spend 2-6 times (considering the average life cycle of an in production software product to be 2-4 years) more money on being maintained than being developed.
Data migration is a massive problem for any organization with data sets at any scale. RDBMS, in general, has gotten in the way of those migrations. People aren't looking at NoSQL just because they cannot sit still but instead are looking to find a better experience with handling data.
I'm not sure if NoSQL is the right answer to that but let's give it a chance and see what happens when people are migrating MongoDB data in 3-4 years.
Integration issues are best handled with good API's. Migration issues are a bigger issue but one thing that good use of RDBMS's give you is the ability to ensure your migrated data is meaningful. Not sure you can do that with KVM-type stores.
large changes? throw away and start over; tools like Rails can help you get up and running really quickly.
I rarely see people go back and fix the mess.