First, you need to unmount your old card from the phone. To get your new microsdhc card ready. Don't use the regular formatting tool from the computer. You should use either the formatting tool in the phone or try this free program on your computer to format the card. I good format job will speed up the card and also speed up your phone and also cause less battery usage.
The name of the program you could download is called SD Formatter 2.0 for SD/SDHC. It works with microsdhc card pretty well.
Here is the link:
http://www.sdcard.org/consumers/formatter/
Once you are done formatting your phone you can transfer all the old files from the old card to the new one. Copy the files and folders first to your computer. Then copy all that you copied back to the new card. Put the microsd card in the phone, turn it on, and everything should work with no problems.