I have done exactly this on two TTGo's recently with the aid of a simple USB card reader.
The most important thing is to ensure that you keep your origional 128 card intact for back-up purposes.
I did not find it necessary to format the card - it just looks like a removeable drive.
Simply plug your origional card into the reader, it should be recognised as a removeable drive and copy the contents into a folder on your desktop.Remove the card and store safely. Insert your 512 card and copy the contents of the folder you made on the desktop and paste into the 512 card.Remove card from reader and insert into TTgo. Switch on and after a few moments reading from the card it should be ready for use.
As a precaution I always use the "safely remove hardware" option when removing cards.