You'd be able to re-defer if you get cash back to invest in a different QOF, but the ten-year timer restarts. But if you lose principal, that would need to be addressed in the QOF's documents; perhaps you'd have grounds for a securities lawsuit and perhaps you wouldn't (if you're invested in a syndicated fund as a limited partner equivalent). If it's your own project, you have to make sure the cash is available to re-invest in a different QOF.