OpenSSH is almost completely compatible with the commercial SSH 1.2.x.
There are, however, a few exceptions that you will need to bear in
mind while upgrading:

1. OpenSSH does not support any patented transport algorithms.

Only 3DES and Blowfish can be selected. This difference may manifest
itself in the ssh command refusing to read its config files.

Solution: Edit /etc/ssh/ssh_config and select a different "Cipher"
option ("3des" or "blowfish").

2. Old versions of commercial SSH encrypt host keys with IDEA

The old versions of SSH used a patented algorithm to encrypt their
/etc/ssh/ssh_host_key

This problem will manifest as sshd not being able to read its host
key.

Solution: You will need to run the *commercial* version of ssh-keygen
on the host's private key:

ssh-keygen -u /etc/ssh/ssh_host_key

3. Incompatible changes to sshd_config format.

OpenSSH extends the sshd_config file format in a number of ways. There
is currently one change which is incompatible with the old.

Commercial SSH controlled logging using the "QuietMode" and
"FascistLogging" directives. OpenSSH introduces a more general set of
logging options "SyslogFacility" and "LogLevel". See the sshd manual
page for details.