I have a db2 instance that was created as a 64 bit instance
arccat
$ db2level
DB21085I Instance "arccat" uses "64" bits and DB2 code release "SQL08026" with
level identifier "03070106".
Informational tokens are "DB2 v8.1.1.120", "s060801", "U808888", and FixPak
"13".
Product is installed at "/usr/opt/db2_08_01".

$ uname -a
AIX archx02 3 5 00C5144D4C00
$

However the application i need to be able to interface to apparently only uses 32 bit instances.

How can I downgrade the instance without having to drop it and recreate it?

I don't think you can... What's wrong with dropping/re-creating it?

The problem is the availability of the unix admin with the root password :-)

What exactly is your problem (error message)? 32-bit applications should be able to directly talk to a 64-bit DB2 instance.