Traveling between one level and the next can be tricky due to the time needed to confirm your admission and create your new I-20. In any case, you will not need to apply for a new F-1 visa simply because you have changed education levels. If your current visa is already expired and you leave the US, it is best to use the new I-20 to apply for a new visa.
I-20s are typically created within the first 2 weeks of the new program. If you will need yours earlier in order to renew your visa, please email us to request special processing.