If you want adjustable transitional surface (“History” must be enabled), I recommend to use “Loft”. There are two ways to do it with “Loft”, depending of your preference.
First, it will be nice to use “Merge” with “Roundness = 0” in order to combine all bottom surfaces into a single one (unfortunately, at the moment “Loft” will not let you use “Chain edges”, unlike “Sweep 2 rails”). “Roundless = 0” will preserve the structure of the surfaces.
Important! If you truly aim G2 blends anywhere, before you even start with the “Merge” command, I recommend to use “Change surface degree” and change the degree of the bottom surfaces from 2 to 3. Then you can use “Merge”, as I described above.
Loft 1: Extract the bottom edge of the upper circle surface and the upper edge of the bottom surface (which you merged prior that). Move these curves away vertically. This will be the G1 (tangency) profiles. Then, make copies of the curves and move them further away. They will be the G2 (curvature) profiles.
Now use “Loft”, adjust the knots with OSnap “Quad” turned on (extremely important!!!) to be in the same vertical location along the X axis, then select “Loose” type and then choose the remaining settings that will best suit your needs. Now you can move vertically one or all of the 4 profile curves to adjust the shape of the loft surface.
Alternative loft 2: Create a closed curve profile that’s perfectly symmetrical (“Snap to grid” is a convenient way to do it), then place it somewhere in-between both surfaces. That horizontal closed curve will be used as an adjustable middle profile, which you can conveniently modify by moving or scaling its control points. Again, use “Loft”, adjust the knots, but this time select “Normal” type and also tick the boxes of “Match start tangent” and “Match end tangent”. Since “History” is on, now comes the fun time with adjusting the middle profile according to your taste.
Finally, no matter which loft type you used, apply “Match surface” to the bottom edge of the loft surface. I found that the following options work good for this particular surface:
Match edges by closest points
Distance = 0.001 units
Tangency = 0.05 degrees
Curvature = 0.05 percent
Isocurve direction adjustment = Automatic
Both loft types are included in the 3dm file below:
Loft works nicely.3dm (9.8 MB)