When source model's connection is changed using following adminapi code;

AdminFactory adminFactory = AdminFactory.getInstance();
Admin admin = adminFactory.createAdmin("admin", "admin".toCharArray(), "mms://localhost:31443");
admin.assignToModel("Sample", 1, "SQLSourceA", "SQLSourceA", "sqlserver", "SQLSourceA_2");

Any jdbc query result would still be from the old data source. However when verified using following adminapi code;

VDB vdb = admin.getVDB("sample", 1);
Model m = vdb.getModel("SQLSourceA");
String jndiName = m.getSourceConnectionJndiName("SQLSourceA");

The jndiName shows that the connection is successfully updated.