Change emailadresses of all users in an organization

Hi,

I need to change a lot of emailaddresses in an Exchange 2013 environment. The company now uses firstname.lastname@domain.com. This has to be changed in firstletter.lastname@domain.com. So for example;

dave.johnson@company.com becomes d.johnson@company.com.

The old emailaddress has to be an alias.

Is there an easy way to do this? There are about 350 users.