Data Encryption in S3 and RDS

I am an AWS user. I want to enable data encryption for S3, EBS and RDS instances.

Question related to S3:

If I enable encryption for my Existing data in S3 What changes I need to do in the Client Application. I am assigning IAM Roles to an Ec2 instance where I have provided permission of corresponding s3. In that Ec2 instance I run an application from where I want to connect to s3 using python boto library. Say for Example, I want to read a.txt file from bucket B of S3. Please inform what change I have to made to get the encrypted Data of S3. How I will put new data that will be Encrypted

Question related to RDS: I want to migrate my Unencrypted RDS (MySql) instance's data to an encrypted RDS (MySQL) Instance. I have read some of AWS Blogs but I have not found any exact answer anywhere regarding the migration from Unencrypted RDS instance to Encrypted RDS Instance.