There have been several questions about changing one or more values in a set of records simultaneously without having to go through the cumbersome task of opening each record and updating the value.
There are different ways of doing this some better than the others; one of them being the best and easiest. Take your pick!
1. Write a Javascript snippet in the form onSave to check if the current record has a particular value and if so, set it to the new value.
Simple way of doing things but the downside is that the value of the record will not be changed until it is opened and saved.
2. Write and run a simple SQL query - whilst this is an easy way of doing it, it surely isnt supported! If you do decide to go ahead with this approach, remember to take a backup of the database before running the query.
3. Write a small program in C# or VB to update the required records. This requires you to be comfortable writing code.
4. Save the best for last! You can use MSCRM 3.0’s very own bulk edit functionality. Run an Advanced find to look for the records that need to be updated. Select Edit from the Actions menu of the query results page. A dialog containing all the fields of the entity pops up and you can make you update here. Cool, ain’t it?!!

If you have any other approaches/suggestions, please leave me a comment!
Take care,
Niths