Could this be a issue? - Adjusting Payment Plan

So, this is something that i was thinking about which i dont know if its something that the Tymit team have though about well in advance but a scenario of:

Lets say you make a few charges to the Tymit Card and you spread them over 3 months. You also make some single month charges to clear at the end of the month. The end of the month arrives and your monthly bill is settled. Lets say you then want to clear the principal balance of the card.

You make the transfer from your current account for the principal balance. Tymit does not recognise this, it will clear the month bill but doesnt recognise the installments. You have to go back and replan the installments yourself. However when you replan you can only change it from 3 months to 1 month. So does Tymit recognise that a installment has already been paid towards this transaction or are you going to end up paying more than the initial transaction?

Similar to this scenario you can replan old transactions within that billing month. This is a cluster of confusion?