SharePoint: How do I keep the old version of custom Workflow?

Hello,

This might be a simple question because I'm pretty new in developing customer workflow.

I used Visual Studio 2010 to create a simple workflow (version 1). If I modify the workflow and deploy it again, the version 1 is gone. The active tasks in version 1 are deleted too. How can I keep the workflow version 1 and its tasks when I deploy a new version of workflow?

Thank you in advance!