I am set to start test planning the upgrade to an existing production system. Testing the upgrade of a system is an area I am not familiar with, and as a result, I am looking for some useful pointers on how best to approach this i.e. the need for baselining data, checklists, pitfalls, reviewing release notes, outstanding defects, fixes to defects etc. etc.

Can anyone recommend some useful and informative links which cover the test planning of upgrades?

Thanks in advance