South Asian speakers have evolved the logical word “prepone” to mean the opposite of “postpone”; to move forward in time. It’s a handy word, but users of it should be aware that those unfamiliar with their dialect will be baffled by this word.
Native speakers use the word “advance” instead of the word “prepone”.
It has now been added to the Oxford and other major dictionaries. So, if one takes that as definition of being English, “prepone” is now an English word.