I asked and got answers - thank you all who tried to help me!

To summarize: if you use PAP with cleartext passwords, it's quite possible
to simply do "mv /etc/ppp/pap-secrets /etc/ppp/chap-secrets" to switch from
PAP authentication to CHAP.

It did not work for me because I used "username xxx remotename yyy" and
had a secrets line like

xxx	*	topsecret

This works with PAP, but not with CHAP. I changed it to
"name xxx remotename yyy" and the secrets line now looks like

xxx	yyy	topsecret

(which is better, of course, if you have more than one PPP connection).
Now CHAP works for me.


Martin