If your client hasn't kept backup before upgrading to SP17 advise them to have patience and wait for SP18. There is no automatic procedure for downgrading the DB, you must make it by hand and it is a nightmare.
All components (application server, webapp, service pages and java client) must have the same version.
The best way, restore from backup prior to sp installation.
rule of thumb, db/client/appsvr/event/agent all must be at same revision to work harmoniously. Reason, patches introduce changes to the db schema (maybe not all, but most) once upgraded the schema has changed.
Once upgraded client/appsvr/event/agent require uninstall to roll back.
Concerning the questions when SP18 will be available, I've been informed by an HP engineer this will be at the end of this month (Februari 2006). In this SP also the unlimited customer fields issue (urging for a lot of users) should be included.
There is no procedure for downgrading. You must do it by hand. Here is my sad story about SP14. I installed SP14 after testing it but there was a bug that I hadn't caught at testing. It was a blocking one (at this time I hadn't found the workaround) so I decided to roll back to SP13.
Step #1 Keep a backup of the current version to another environment. Step #2 Restore the backup you had before the upgrade (If you hadn't one, sorry to say but you are screwed!) Step #3 Locate the items that were created or modified AFTER the upgrading (the easy part) Step #4 Export them (nightmare, especially if you have relations) from the system with the newer version Step #5 Import them to the system with the older version (man, THIS IS NIGHTMARE) and pray to your $DEITY that this will work.
However if you don't care about the items created or modified after the upgrade, just restore the db backup you had before the upgrade and of course install again all components to the version before the upgrade.
As I said earlier If you haven't a backup before the upgrade you are in deep s**t.