I would go down the "remember" lane: remember the selections just before cancelling the bets (don't forget to choose the option "Remember selections' bets", then cancel the bets and place new ones using the bu_backp of the remembered selections as a reference.
"cancel then back" would also work in your case, but in order to make it work in Test Mode you will have to turn on "Delay placing and cancelling Test Bets" in the "Test Mode" settings tab, as otherwise test bets will be cancelled immediately, unlike real bets.