Pega version 7.x upgrade documentation is the wrong place to be discussing the benefits of split schemas. All upgrade documentation should simply read, If you have single schema follow path A, If you have dual schema follow path B.
Moving to split schema should be a separate enhancement and has nothing to do with the update/migration you are trying to preform. In fact it just adds in a confusion factor . Pega own docs contradicted itself.
Planning your upgrade
The Pega 7 Platform supports several configuration options that can affect the choices that you make during the upgrade. Before beginning, read this section thoroughly.
l Do not change your environment during an upgrade. For example, if you are making changes to your application server, or database server, do so before you upgrade the Pega 7 Platform.
lChoose a configuration type: single-schema or split-schema system. Pegasystems recommends a split-schema system. See Split-schema and single-schema systems.