Yes, mine is a retrofit.

I hadn't thought about adding the extra layer of GWB. The problems I see with doing this is extra work around windows and door openings, and adjusting all the elctrical outlets and switch boxes to the right depth.

Should I be worried about moisture absorbing into the MDF with this on the floor line?

oh, no. I am SOL. the foor was installed after the moldings, so I have a gap of 1/2 moldings plus the 1/4 expansion gap. Once the moldings are remove I will have a 3/4 gap all arround. If I trim the base of the 5/8 drywall, I will have 1 3/8 gap to deal with.

Options:
1)What about a matching maple 1 1/8 thick boarder around the flooring?
Expansion issues? Look? etc?

2)Add matching wood planks since the floor is enginneerd should match perfectly.
Removing/Adding tongue grove glued 3 planks cleanly
How to handle where wood is perpendicular to the wall

3)Install new flooring 1928sq/ft
expensive

4)Give up ideal look and go with 4 or 6 3/4 square MDF moldings
may not look right with the the minimal jam Tre-Piu doors I ordered.

Wow, ThreeDogKnight...

Your diagram is really nice. I think I'll print it out and show it to my builder. If the wall and the MDF are both painted white, then the difference in materials would be invisible for the most part.

Our builder wanted to do this layered molding as shown in the above photo where they put in the first layer, then put in the floor and then add the shoe molding. I strongly objected to this and he said he really didn't want his guys putting in the floor first. Why is this a problem?

'tooth, it seems to me like the best aesthetic option would be the border around the perimeter of the floor either with a contrasting material or matching. It'll be a lot of tedious cutting and fitting. At the perdendicular ends, I would run the border parallel to the wall and make neat miters at all the angles.

Are you handy yourself or contracting work?

Oh, BTW, 3dog, if you're concernd about water, ther's an exterior MDF called Medex. I think Roseburg Fir makes it.