What is the alternate to SA account in SQL Server?
On your development server, you can grant the standard users the db_datawriter and db_datareader roles. You probably want to grant the db_owner role to the power user. As these are database level roles, you have to repeat this for each of your databases. Regarding the web server, you should create a limited user that only has been granted the db_datareader role.

As the sa login is hardcoded to be a member of the sysadmin server, you can't limit the permission of this account. The best way to migrate is to replace all the connection string you can find and then finally disable the sa account.