Q. I BitLocker encrypted my disk with used disk space only switch. Can I now encrypt the rest of the disk?
A. The option to only encrypt used disk space speeds up the BitLocker encryption process by only encryption blocks actually written to however if there was previously data on the block then that "empty" area would not be encrypted potentially leaving the content readable using certain disk tools.
If you later wanted to encrypt the empty space you can encrypt with the WipeFreeSpace switch (https://technet.microsoft.com/de-de/library/jj647761(v=ws.11).aspx), e.g.
manage-bde -w <disk>: