Hi David,

This reply maybe too late but I had the same question after creating a custom table and realising I needed a couple more columns. I already have the table on my dev environment, but not fully into live, and using migrations will keep everything in sync.

Here is an example extending the code from https://our.umbraco.com/documentation/Extending/database/ You can add extra code into the same class which is good for simple changes like this.

Start by adding an extra step to the Initialize method, which specifies a migration to follow on from the first step.

 ... 

  // Each step in the migration adds a unique value
  migrationPlan.From(string.Empty).To<AddCommentsTable>("blogcomments-db");

  // Add a second step
  migrationPlan.From("blogcomments-db").To<AddExtraColumns>("blogcomments-AddExtraColumns");

 ...

This keeps track of which migrations have been run on the current database. It's like a code version of dbUp or roundhousE, but it chains the migrations together to keep things in order.

Hot Tip: Look in the umbracoKeyValue database table to check the last migration step that was run. It stores the magic strings from this code, so for example if you used the example code successfully there will be a row for Umbraco.Core.Upgrader.State+BlogComments with a value of blogcomments-db.

Next create a new method, which has the migration commands you need. Here's how I added two columns:

 public class AddExtraColumns : MigrationBase
    {
        public AddExtraColumns(IMigrationContext context) : base(context) {}

        public override void Migrate()
        {
            Logger.Debug<AddSignupSheetTable>("Running migration {MigrationStep}", "AddExtraColumns");

            if (!ColumnExists("BlogComments","Location"))
            {
                Create.Column("Location").OnTable("BlogComments").AsString().Nullable().Do();
                Create.Column("SessionId").OnTable("BlogComments").AsString(50).Nullable().Do();
            }
            else
            {
                Logger.Debug<AddSignupSheetTable>("Additional columns (checked for Location) already exist, skipping migration");
            }
        }
    }

Hope this helps!