Assuming for a second that your can't cast spells of a higher level than slot you have. You start all your silly hand waving and praying for a max level spell, the slot is techanly used for the day, but before you complete your masterpiece, you decide to make it bigger with DMM (heighten). Now DMM and Heighten care about only two things, do you have enough turn attempts for the level increase and is the spell below level nine. So the two feats agree, the spell is a legal target, you burn two turn attempts to increase the spell by one level.
Now...does it fizzle or does it get cast?