Customers do not have to engage with a solution partner to upgrade their system. Atlassian takes on the cost of any software or hardware upgrades.
Atlassian is constantly improving, testing and releasing new features to keep up with best security practices and constantly changing user expectations. As the changes in the cloud are done in small steps at a time, but in several steps. Also, in case of big changes, customers who are in Enterprise Plan and who enabled Release Tracks feature get a list of planned releases 2 weeks in advance.