Answer below assumes you mean ownership of configuration objects within Splunk. If you mean something else (e.g. OS-level file ownership of configuration files) then this may not apply.
If you have command line access to system and read/write access to the configuration files, the easiest option is to modify all of the local.meta files and replace references to the local account with the AD account. Doing so will require you to restart Splunk for the changes to take effect.
Alternatively, it is possible to do via the REST API, but you'll need to enumerate all of the configuration objects owned by that user and then update each one individually to the new user.
