Succession Planning

I think succession planning need not be done when you decide to join another company. It should be done from day one because you wont be stagnant in your position - no one will!

My response to the questions would be:
Opening statement: Succession planning should be done from day 1 an employee starts on a project or a company because nobody stagnates in their role - people move to different roles within a company if not joining a different one.
Next part: Yes (never say yeah or haan), I do feel it is very important.
How, planning part: In every project I have worked, I created a learnings document and I spent not more then 5-10 minutes in my day to put down the important stuff I learned about the project or the domain or the client. I would also create another document that details the test environments - URLs, files etc - DB access details, test case location details and all such things that a QA would need in order to carry out the work. so when a new QA hops on the team, I would ask them to read these 2 documents after he/she completes reading FSR (requirements doc.). This way knowledge is not lost when a person moves from current role to a different role.