Walla does not accommodate migration of plans with a future start date sold in your previous software. In the time leading up to migration you’ll want to record any memberships or plans with a future start date sold to clients.
Once your migration is completed, you will need to sell the client their plan and discount their payment or first payment in their recurring plan and then adjust their next payment.
Note, you will not be able to edit the start date of a plan with a 4-week billing cycle. To accommodate this client, you can issue comp passes or a non-renewing plan until their next billing cycle. At that time you can sell them a new plan at their regular billing rate.