First rule of Commvault is "do not delete your client". If you delete your clients, the backups go as well (including extended / long term retention etc).
Since you want to age out your disk libraries and not your tapes, assuming you will not be doing any further backups on the existing clients, you simply need to Deconfigure all your clients.
Once clients are deconfigured, they will honour days only retention rather than cycles which means jobs on disk "Primary" copies will age out as the days pass irrespective of their cycles requirements.
Another method is to manually go through your disk Primary copies and delete the jobs from the job history.
Once all jobs have aged out from your disk libraries, you can safely deconfigure the libraries in Commvault (which will prune out the data on them immediately as well).
Do not deconfigure or delete your tape libraries, or delete the jobs on the tape storage policy copies. Let them gracefully age out over time based on their retention days (which I assume would be much longer than the disk ones).
Obviously, you need to keep your Commserve around and possibly 1 MediaAgent for restore and data aging purposes.
Hope this helps explain things.