The existing module is capable of handling 3 d secure without any changes.
You are also correct in chnaging the hidden TxType field to 'AUTHENTICATE' to start using their new service AUTHENTICATE and AUTHORISE
But please note this system only goes live on 1st of August, so don't make any changes now unless you want to test it with the live server.
$plain .= "Apply3DSecure=0" . "&";
Rgarding the above code it is only an option. If you set up the 3d secure rule in the admin part of protx (which is much easier to do)
you won't need it.
Some one mentioned about redid the protx module to work with the ver 2.22 protocol, and also to work with 3d secure. I am not sure why??. The existing one is more than capable of doing the job.
IF it is a question of Authenticate and Authorise, it is just a matter of changing one TXType field to "AUTHENTICATE" from DEFERRED or PREAUTH (depends what you are using at the moment)
Hope it helps.