I'd remove all shingles from the over-deck portion of the roof, then start from the bottom edge with new felt and 3-tab shingles. When you get to the first course of T-lock shingles, slide the felt and 3-tabs under. Nail, and then use roofing cement to seal the joint between the T-lock shingles and the new 3-tabs. This connection would be right about at that gutter line.
Oh, and add metal drip-edge to all edges of that over-deck roof. When you cement the edge-line shingles to that, it makes for a stronger, more wind-resistant edge. Plus, it should help keep wind-driven water from damaging your sheathing.
That's what I would do anyway!