the rules look like there are 5 days to rearrange the match at either ground. After that it's a forfeit to the team that can't play.
However:
(c) The National Game Board shall have the power to vary these arrangements if it is deemed necessary.
Who knows. FA Cup seems to be allowing rearrangements now, as is the League Cup, but both are at later stages (I think it's in the rules that a League Cup semi can be rearranged).