I have two Amazon S3 buckets: Bucket1 and Bucket2. Bucket1 is currently used as the main storage method with about 25 GB of files.

If I change the storage method to Bucket2 and move all files, will the files be transferred between the buckets or from Bucket1 to my server then to Bucket2? I’m trying to calculate the cost of the move, and if my calculations are correct I’m looking at below $3.

The files are fetched (i.e. transferred from bucket 1 to your server) and then pushed to the new storage location (i.e. your server to bucket 2).

If you have a way to move them from bucket 1 to bucket 2 yourself manually (presumably cheaper), then when you change the storage configuration options you can check the box that says "I've already moved my files" to prevent that behavior.