Plans cannot be converted or upgraded after purchase. Each account type is a separate product with its own rules and drawdown mechanics. If you'd like to try a different plan, you would need to purchase a new account.
This includes moving between Flex and Daily Payouts. You can hold both at the same time, since account limits are counted separately for each product type. If you are weighing the two up, see Daily Payouts or Flex Challenge: which should I choose?