I’ve just started messing around with the Merge tool to combine my meshes into one big mesh. I was wondering if it’s possible to change an item after it’s been merged. For instance, say I have 3 planes that are evenly spaced. I merge the 3 planes into a new mesh. Is it possible to change the width of the plane at the center and have that reflected in the new mesh? I’d love it if I could change the size of the item at the center and have it push the other items further away but I figure that the only way to do that is to keep my planes unmerged and just move the items if the center one changes.

Yes, but its in your hands.
you need before merge a plane note the indices length. merge your plane, note the indices length. if you would want update the plane, with exact same faces amount, simply loop over these vertices and multiply by the factor you want.
if you want replace or remove the plane, simply rebuild the buffers with the offset. If you have more planes that you want to update, make sure all your saved indices are updated as well.

ok cool thanks Fabrice!