I just tried this myself with the same result.
A default Elastix 2.4 runs on FreePBX 2.8.
I went to the unembedded FreepBX and ran the 2.9 upgrade process.
Rebooted, and now it shows 2.9.0.15.
The scripts gives the same error.
[root@3227-pbx ~]# curl -s https://convert.freepbx.org | bash
Checking that 'curl' exists ... OK!
Validating sha256 integrity ... OK!
Trying to download converter to /tmp/tmp.XghJhY6651 ... Complete!
Validating download ... OK!
Starting FreePBX Converter version release/20151217r1
Testing connectivity to Conversion server...Success!
FreePBX Conversion Wizard
-------------------------
The FreePBX Conversion Wizard needs to be run on two machines, the NEW
machine, which must be an ACTIVATED FreePBX Distro machine, and then it must
be run on the DONOR machine.
The DONOR machine is the machine that is currently processing calls, and is
the machine that will be migrated to the NEW machine. No changes will be made
to the DONOR machine, and this script will not stop or restart any services
that may cause an outage.
If this is the NEW machine, just push 'Enter' to prepare this machine
Enter ID (blank if this is NEW): c...............6
Testing FreePBX functionality ... Failed. Received '/etc/freepbx.conf doesn't exist.
This must be run on a FreePBX machine' instead of 'OK'
Cleaning up...Done!
[root@3227-pbx ~]# ls /etc/free*
ls: /etc/free*: No such file or directory
[root@3227-pbx ~]#