There is no way in Tixigo to move just some tickets of an order to another event (date). You can move the entire order or the entire group only.
There is a trick you can do to sort of give you that feature but it has one drawback: once moved you cannot refund the tickets you moved with Tixigo, you would have to manage that manually.
The trick is to cancel the tickets you want to move and select No Refund. This way you cancel tickets but keep all the money. Then you create a new order for the new event (date) using the merchant's ticket office (within the calendar page of the portal) and select the same tickets you cancelled before but you select a Payment Type that is anything other than OnLine. We added a new payment type named "Credit" exactly for this use case which means that the payment comes from a credit elsewhere.
Be aware that Tixigo allows you to use different payment types but only manages money for payments identified as Online (using Stripe). All transactions marked with any other payment type are simply stored in the database but it's up to you to manage the money or their meaning for accounting. So the "credit" type is just a proposal for this trick and all sales using this type should simply be ignored when you do your accounting.