2540638 - Cannot downgrade ASE from 16 to 15.x using sp_downgrade - SAP ASE

Got one of the errors below when running sp_downgrade to downgrade ASE from 16 to 15.x

sp_downgrade
go
Msg 2812, Level 16, State 5:
Server 'xxxx', Line 1:
Stored procedure 'sp_downgrade' not found. Specify owner.objectname or use sp_help to check whether the object exists (sp_help may produce lots of output).

OR

sp_downgrade 'prepare',@toversion = '1503' , @verbose = 1
go
'15.0.3.' is not a valid downgrade version.

SAP Adaptive Server Enterprise (ASE) 16. Any SP / PL combination.

sp_downgrade, not found, Msg 2812, 15.x , KBA , BC-SYB-ASE , Sybase ASE Database Platform (non Business Suite) , BC-SYB-ASE-CE , ASE Cluster Edition (Standalone) , Problem