on Windows you probably would need to shutdown ASE, rename the log file, and reboot ASE.
Alternatively, run sp_errorlog twice, eg:
================
sp_errorlog 'change log', <new_file_name>
go
--
-- out in OS rename/move <old_file_name>
--
sp_errorlog 'change log',<old_file_name>
go
================
This should get you back to using the original log file name, which also means there's no need to update the RUNserver file (or Windows registry) with a new log file name (ie, next time the server is started it'll continue to use the 'normal' name).
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Obviously (?) if there is a process that monitors the errorlog it will need to be checked to make sure a) it knows how to switch over to a new log file after executing sp_errorlog or b) it is reconfigured/restarted to find the new log file.