In answering many of the questions, you may notice that I vary between the Geek Answer and the Suit Answer. The Geek will take advantage of open source to do everything in house, relying on his own capabilities. The Suit is often more inclined to outsource solutions, unless he is convinced that in-house staff can handle them. Suits may figure that it is easier to change vendors than to hire/train employees.
The full answer to your question lies somewhere between the this answer and this answer. The short answer is, if you are changing only one component in a stack that works, then you will need to tune and test to make sure that the upgrade hasn't broken anything. This is the in-house solution. The supplier of the stack component you are changing may be able to tell you whether it has been tested with the configuration you are using, but always verify for yourself.
If you don't believe you have the necessarily capability in-house, then there are vendors who will sell you stacks that are guaranteed to work properly; these vendors will also sell you upgrade and support plans to cover problems along the way and supply improvements as they become available.
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