Under prior law, this was a way to save tax dollars if the value of your Roth IRA declines after a conversion.
Under prior law, the recharacterization rules made it possible to undo a Roth conversion that turned out to be undesirable due to a change in circumstances, including a loss of value for investments held in the Roth. The owner could then reconvert to a Roth at the lower value and therefore a lower tax cost. A change in the law makes this strategy unavailable. The recharacterization rules can no longer be used to undo conversions.