If you are downgrading to Adaptive Server versions 15.0 or 15.0.1, run installjsdb script from the earlier version.
1> sp_configure "enable job scheduler", 0 2> go 1> sybmgmtdb..sp_sjobcontrol @name=NULL, @option="stop_js" 2> go
isql –Usa –Psa_password –Sservername –i%SYBASE%\%SYBASE_ASE%\scripts\installjsdb
sp_configure "enable job scheduler", 1
1> use sybmgmtdb 2> go 1> sp_sjobcontrol @name=NULL, @option="start_js" 2> go