What OS ?If its Unix What Shell you are using ?

C Shell:

Go to Profile of Oracle userid thats .cshrc and make an entry like this
And/OR
at the end of Oracle provided file "/usr/bin/coraenv" under local modifications

setenv TNS_ADMIN=/O_H_817/network/admin

K Shell:

Go to Profile of Oracle userid thats .cshrc and make an entry like this
And/OR
at the end of Oracle provided file "/usr/bin/oraenv" under local modifications

set TNS_ADMIN=/O_H_817/network/admin

Rename(prefix) Oracle 806 binaries to say .old

lsnrctl as lsnrctl.old so that it never gets invoked and always 817 binaries will be invoked.