Nope, this is indeed how you can phaseout storage policies.
@drPhil , one point to remember is to monitor the backups on the older Storage Policies. they should age off as new backups run to the new Storage Policies, but as life dictates, the more you do something, the more likely at least one of those attempts will have an issue.
Just keep an eye on the older backups using the Data Retention forecast and Compliance Report and ensure that they age off in due time.
Remember to set retention for the old Storage Policy to 0, or data will be retained by Basic cycles rule