Hi,
i have a sybase 12.5.x dump. i wanted to load it into a sybase 15.0.x.

Jul 01 09:53:57 2009: Backup Server: 4.132.1.1: Attempting to open byte stream device: 'compress::\Backup\restore\dump.dmp::00'
Jul 01 09:53:57 2009: Backup Server: 4.177.2.1: The database page size of 4096 bytes obtained from ASE is different from the database page size of 2048 bytes read from the dump header. The LOAD session must exit.
Jul 01 09:53:57 2009: Backup Server: 1.14.2.2: Unrecoverable I/O or volume error. This DUMP or LOAD session must exit.

is it because i have setup the server or the database wrongly with the different pagesize?

is there any way i can remedy without re-installing sybase?

Yes you must use same pagesize
recreate the instance using srvbuild

use bcp to init the data for your migration, or change the page size
I suggest using bcp and leave your pagesize at 4096.