Download Astro file manager from the market first. Put your old SD card in your phone with the music on it and transfer it (cut and paste by longpressing on the file to move) to a folder on the phone memory itself. Once you have done that put your new SD card in the phone and transfer it back to that.
You need to use the phone hard drive as temporary storage during transfer from one to the other. Thats the only way i can think to do it