How upgrading the pledge manager affects the installments in a gamefound boardgame campaign?

So i pledged for This war of mine 2nd edition Boardgame, i used stretch pay, and already paid 3 installments, and a few days ago i saw the added a "new" expansion that wasnt there before, so when i tried to add it to my pledge it updated the monthly payments to a new higher cost of course, and during check outm it does shows the new total minus the amount i already paid, but when im about to paid it just show the total followed by the message "You're about to pay 1 of 7 installments of (new higher price)" with the new updated price (like if im were starting from zero again but with a higer price each month), since im new to gamefound and pledging, im a bit confused im doing something wrong ? like i need to buy the new expansion separetly ? i need to wait till i finish all the installments so i wont "start" over when i already paid 3 installments ? or i would indeed need to pay all over again to add this new expansion ?

Thanks in advance