bsc修改了defaultEpochLength以后，会导致从头验证区块异常，

func (p *Parlia) verifyHeader(chain consensus.ChainHeaderReader, header *types.Header, parents []*types.Header) error {
	
	// Ensure that the extra-data contains a signer list on checkpoint, but none otherwise
	signersBytes := getValidatorBytesFromHeader(header, p.chainConfig, epochLength)
	isEpoch := number%epochLength == 0
	if !isEpoch && len(signersBytes) != 0 {
		return errExtraValidators
	}
	if isEpoch && len(signersBytes) == 0 {
		return errInvalidSpanValidators
	}

验证头部header和epoch是关系的，所以不能任意修改defaultEpochLength,需要采用硬分叉升级