That's a humongous mistake if you do that. You cannot transfer a UDP line's upgrade to a TDP line and keep UDP. That ended August 25th.
Do not be fooled by people spouting off here saying you can do it; you cannot and they are trying to lure others into doing it to see what will happen so they will know for themselves and you will be screwed not them. They have never done it themselves, and until someone posts screen shots showing what happens now you should not attempt this. Do not fall for that nonsense. There is no reason to believe this still works and you can read my explanation as to why
here.
However, the Best Buy Method still works. You don't need to downgrade a UDP line to use the Best Buy Method.
The Best Buy Method is the ONLY method that still works after the 24th of August and does NOT require you to give up a UDP line.