Will I still need the S2 remote if I bought an upgrade kit from S2 to S3?

No, you do not! There is full compatibility between the S2 and S3 remotes.

You can connect both the S2 and S3 remotes to the new S3 controller. This gives you the flexibility to choose which remote you want to use, or you can configure both remotes to perform the same functions. The choice is entirely yours!