Impact of changing instances' passwords As part of a security policy, the company I work for has given me the task of changing the sa's and SQL Server Agent service accounts' passwords for several instances.

What's the possible impact of doing this? Is there anything in particular I should watch out for? Is there any whitepaper, guide or even blog post about this kind of task? What kind of 'backup' plan should I have in place in case anything goes wrong?

Any guidance is appreciated.
Well for security reason you should not be letting anyone use the "sa" account. If you do then obviously changing that password will break whatever is using it. I try to set this password to a random password, rename it (only SQL 2005 and above allow this), and then disable it. I have not found anything so far that explicitly requires the "sa" account to function, my own admin account suffices.

With regards to the service account this should not cause any problems. Again as long as no other resource or service is using the same account.

The only backup plan I can suggest is having the old password handy to reset the accounts back to in the event something breaks.