Send the check to TA and they will mail you the common.
Keep the preferred certificate because TA and CDFT have the master record.
It's that simple.
If you have not sent the money already, you may want to call the TA. I am hearing you can save a few bucks if include a self addressed envelope and notify them you are converting preferred D (if that is what you have), because lamlam said he spoke with TA and it is cheaper.
I sent an email to TA inquiring what the price would be if we were converting one preferred D and if we included a self addressed envelope.
I already mailed my check, but if it is significantly cheaper, I may make other arrangements. I'll post what I can find out.