The 14" may be fine with 3 of the current supplies switched to a new supply trunk.
Or, it could be that the slowr air flow in the existing 14" will make those remaining supplies lag behind in cooling and heating.
I can't tell from here, if those remaining 4 supplies should be moving 250CFM or 460CFM.
Thats why on site testing of static is important. It can eliminate/reduce the guess work. And prevent having to pay for repairs/alterations twice.
The supplies should be check for how much air they are currently moving.