Instalment Payments

Hello all, 

We have a 5 instalment payment option for subscriptions.Our payment type is set-up as an invoice. 

I have encountered a probelm with changing a credit card in an invoice schedule.  The new card was entered in constituent record/transaction tab and then selected and saved in the payment schedule.  The new card was correctly charged last month.  However when I sent out reminder letters for this month's instalment (with the last four numbers of credit card in letter) the credit card number "picked-up and mail-merged" (for the above customer with the new card) was the old card NOT the new card.  If I can't clear the old card from the payment schedule, nor  deactivate the card in the constituent record/transaction tab how do I get the correct card (last 4 digits) to pull out for a mail merged letter?.    Does anyone have any advice with this, please?