Any pledge that is listed on the pledge page as being payable in installments is - as far as I know - also payable that way to upgrade.
If you want to make payments on a pledge that is not described for monthly payments, then you should submit a ticket to Support here:
https://seforums.pantheonmmo.com/support/open.php
Tell them what you want and what you wish to pay and the relevant people at VR will give you an answer.
See ya on Terminus