Prevent expiration of existing Backups

Dear Gurus,

We have a policy atatched with an SLP. Many clients are using this policy and we already have many existing backups for them. The current retention for the SLP is 1 year. We plan to change the retention to 6 months. The issue is there are some clients for which we want to keep the backup for 1 year (for the existing backups; for new backups of these clients requiring 1 year retention, we will be creating new SLP and attach it to the existing policy). So would like to know how to tackle this? What are the alternates available to us (for exmaple: duplication, new SLP etc.) and what are the implications/impact of each of them. We don't mind in creating new policy and/or SLP for this purpose.

Thanks.