How does one add a new base field to a content entity? I'm currently porting my contributed module to Drupal 8, and sometimes I realize I've chosen the wrong field type. I also know that I will need to add additional fields later on, and it would be nice to avoid having to uninstall and reinstall the module every time I do this.

I looked at Add field to existing content entity, but it did not seem to be the actual question.

My question is basically: what's the equivalent of similar D7 update.php code with D8 and entities?

&lt;?php
function MYMODULE_update_7101() {
  db_add_field('mymodule_table', 'some_new_field', $field_schema);
}
?&gt;
Adding fields just works. Define the new field, run update.php, it will compare the new and existing definitions and figure out that there's a new field to create.

Renaming/deleting fields works too, but only when you have no data. When you have data, then you will still need to write an update hook, or a migration. We're missing documentation on how that will look exactly, but basically, you use the same schema functions as in 7.x and then update the last installed field definitions.