Can I upgrade schemas?

**Expected Behaviour**

I’ve wrote schema setting on serverless.yml, and I’ve created one table by running migrate command.
After that, I wanna update schema about add GSI on that table, so I added settings about GSI on serverless.yml, and executed migrate command.

**Actual Behaviour**

But, not expected, table’s schema remains same state.

I can see below logs.
> DynamoDB - Warn - table [tablename] already exists

Can I upgrade schemas, not deleting exsiting table, like `sls deploy`?

Start by comparing the schema settings in serverless.yml with the behavior of the migrate command and the existing sls deploy flow. Define done as updating an existing table's schema, including a new GSI, without deleting the table or leaving the DynamoDB - Warn - table already exists message as the only result.

Written by the indexing model from the issue text.