Possibly. Licences are by seat, Atlassian don't really care how you allocated them. If you want to drop a "client", assuming that means "our client has 5 accounts in our system", then just disable the 5 users and enable the users for the new client.
If you're talking more about having a JIRA/Confluence set dedicated to a client (you run it, but they are the only users), then it depends on what you're planning to do. If you are going to completely remove the system from them and re-purpose it for a new client, yes, you can reuse the licence (because it's your system, not theirs), but if you plan to leave it running, then no, you'll need a new licence for the new system (although you can simply let the old one lapse when it naturally expires)
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