Users usually consider changes in the UI of a product as very negative. Two examples:
The Facebook's timeline (with its two columns display of the events) was subject to controversy for a long time until some users understood the benefit of it, and others just accept it,
The ribbon in Microsoft Office products raised a huge number of angry voices from users who wanted their old menus back.
Such reaction is due to the disruptive nature of those changes and the fact that users should spend some time to learn the new UI. Instead, one can change the UX by making small, not particularly disruptive changes, moving step by step to the new way of interacting with the product. Google Chrome, Gmail, Google Maps or Google Search are a few examples of this approach: while all those products had changed substantially over the years, they did it incrementally with no major, disruptive changes in the way the users interact with those products.
The frequent incremental changes, on the other hand, have their drawback as well: the users can be pissed off by the fact that they should constantly adapt themselves to the changes in the UI. For instance, the introduction of hotdog icon in Google Maps lead some people, including me, to think that there is no way any longer to get the terrain view, which was previously available as a button on the main screen.
When I need to improve the UI of a product, what should I take in mind to decide between a disruptive big change and a series of small incremental changes? What should I do to make the move less painful for the users?