I have a new customer who hasn't chosen a domain name yet but I need to start development for him using a subdomain of my domain. I'm on Bluehost using add-on domains and subfolders. Is there a way to convert the permalinks in the content easily? Can they be changed with SQL once he's selected a domain name?
You can use the Search and Replace plugin to fix the URLs once the new domain is chosen.
Likewise, plugins such as BackupBuddy (paid) or Duplicator (free) -- which are very useful for backup, possibly coupled with migration to another server or domain -- also perform this search-and-replace operation.