Answer
On an on-premise, WebLogic-based Middleware Server, you can decrypt the existing password directly rather than reinstalling: from the Application Server host, open a command line in mw_home\mws\oracle_common\common\bin and run wlst.cmd to start a WebLogic Scripting Tool session. Once loaded, set your domain path, then load the encryption service via weblogic.security.internal.SerializedSystemIni.getEncryptionService(domain) followed by weblogic.security.internal.encryption.ClearOrEncryptedService(service). Separately, open the domain's config.xml file and search for the AES-encrypted password inside the node-manager-password-encrypted tag, copy it, then decrypt it in the WLST session with print encryption.decrypt('<EncryptedPassword>') — this outputs the plain-text password you can use to log in, after which it's worth resetting it to something memorable. This technique isn't IFS-specific — it's a standard WebLogic Server administration approach available on any WebLogic instance — but decrypting it this way does require direct access to the Application Server host itself, so it should be restricted to system administrators with that level of access.