The docs say:

Pinot only allows adding new columns to the schema. In order to drop a column, change the column name or data type, a new table has to be created.

Does this also mean that moving a field i.e. from dimensions to metrics is not possible?

If the data type doesn’t change, then pinot won’t throw exception, but your default null value will change e.g. INT default null value is INTEGER.NEGATIVE_MAX for dimension field and 0 for metric field.

So if I apply the schema with the moved field and reload the tables older segments will continue to work? The consuming realtime segments will work too?

Yes, just the default null value won’t change for persisted segments.