I suppose you will be going for the latest 15.0.2 version of ASE. if so, you will have to install the latest EBF as well, as there is a bug in 15.02 which results in stack trace when statement cache is used.
Additionally, sometimes it is better to delete the table stats and then create them, as I have seen some issues here.
if you have any extended SPs from ed-barlow etc installed then some of them do not work the same with 15 as there is a lot of changes to the system tables, some of the inbuilt functions have been deprecated and so have some trace flags. no show stoppers.
aside the above I dont see why you should have any issues whilst upgrade.
Anyone know what to look out for on the above upgrade? A link or two would be fine as well (couldn't find anything useful when I Googled).
I went from 12.5.4 to 15.GA It was failry painless.. be aware that 'order by' with 'group by' is now SQL 92 enforced. I'm now at 15.0.2 .. .ASE15.0.3 is now available from Sybase (made available this week).
If you are going from-to the same page size you can do big endian dump-load .. across platforms ..otherwise bcp or sybmigrate (with it's caveats does a decent job). I wrote my own migration utilitiy set .. not a fan of sybmigrate, just my personal preference.
Afterwards I would performance test and tweak the environment. Out of the box I observed huge performance improvments.
Been up for over a eyar now with zero downtime. This is a very stable release.
Also, be aware that the new optimizer for 15 is generally good but has a few bugs. especially when using join conditions, say you join table A & B based on some criteria and then check for some condition in table B, then wat the optimizer does is instead of checking the table B once and getting the values it performs the checks in a loop,
example: update A set ...... from A JOIN B on A.xxx - B.xxx and B.yyy in ('dfas', 'fff')
this would run very slow, the workaround will be
select .. .... ... into #BB from A where yyy in ( dfas', 'fff')