A new Custom Channel Group may be applied retroactively to organize data that has been previously collected.
When you create a Custom Channel Grouping at the user level or create a new Channel Grouping in a view, you:
- Can immediately select it in reports.
- Can apply it retroactively and see historical data classified by your new channel definitions.
- Change how reports display your data, without changing the data itself.
Read more here: https://support.google.com/analytics/answer/6010097