Install oracle 8.1.7 on a different dir
copy 8.1.5 init.ora to 8.1.7 Oracle_home/dbs dir
change the oracle_home of the sid running in 8.1.5 to
8.1.7 oracle_home in /etc/oratab file

shutdown the 8.1.5 database
set the oraenv of the sid which is running in 8.1.5
startup the database using svrmgrl

run the script U0801050.sql which is in $ORACLE_HOME/rdbms/admin dir
which will take care of upgrade

Thats it, you are done.,

All the best