No, it isn't possible. You cannot change the page size of an existing server.
You must create a new server with the desired page size and migrate the data over.
There is a utility tool for doing so, called SybMigrate. See http://infocenter.sybase.com/help/topic/com.sybase.help.doc.ase_12.5.3.NF1253a_bulletin/html/NF1253a_bulletin/NF1253a_bulletin42.htm