1) When one phone uses SD card... he puts information in it ( Apps/Data/Chace and etc...) If you unplug the old card... the phone will start to search the missing files from your old SD card.
2) When you plug NEW SD card into your phone... that must be done when the phone is OFF (you may break the SD's if the phone is ON)
3) I will advice you to save your data (pics, music and whatever you have on your OLD sd card) and FACTORY reset that phone
When you plug the new card on your restarted phone it should be fine. I do not reccomend to use different SD's on your phone without restarting your data on in.