A common failure you might encounter, even after going through the pre-upgrade tasks, is that the log may get full due to the catalog changes that are being done during upgraded. If so, log in via isql to the new server, and issue:
isql> dump tran dbname with no_log
This will free up the log space, and allow the upgrade process to continue.
In some cases, the space estimations done by preupgrade might be insufficient for the data copy phase of upgrade. In that case, you may get an error that there is insufficient space in the system segment for the upgrade step. The upgrade process will hang, waiting for space to be provided. You can log in to the new server using isql, and use alter database to increase the size of the database.
isql> alter database dbname on device_name = "2m"
In 12.5 Adaptive Server, alter database allows you to specify the size to alter the database with the unit-specifier 'm', or 'M'.