Full parts layout.
The left side is the Assault parts, the right side is the Buster parts, and the center section is the shared upgrade parts.

V2 Assault Buster Gundam – size comparison with a cigarette pack

Upgrade parts for the main body.
To attach the Assault/Buster parts, you'll need to swap out various joints and armor pieces on the MG V2 Gundam Ver.Ka.

Starting with the arms — you'll need to replace the shoulder's inner frame and the upper arm skeleton entirely.

The shoulder frame is swapped out for a part with a small additional slot, so it can accommodate the Assault shoulder armor.

To support the Assault shoulder armor,
the shoulder is replaced with a fixed-type piece (right) that has the movable section removed.

For the elbow joint, you need to swap from the ring-style method of slotting the upper arm into the elbow
to a pre-assembled form where the upper arm and elbow come joined together as shown below.
The old ring-style connection had a tendency to pop apart during posing, so this is a nice improvement.

The armor on the underside of the Booster is also replaced with a piece that has slots cut in, so the Buster weapons can be attached.

The inner frame that holds the shoulder in place is also replaced.

The new frame has a more protruding shoulder peg so it can accommodate the Assault shoulder armor as well.

On top of that, two brand-new locking parts are included that weren't there before.

These parts lock the center of the hip joint plate in place.
Previously, the hip joint plates on both sides were designed to spread outward during transformation,
which often caused them to splay open unintentionally in MS mode and throw off the pose.
Locking that section down tight has made things a lot more stable.

Only 2 leftover parts.

Limb articulation is similar to the base kit,
though the Assault front skirt armor makes it a bit harder to raise the thighs.

V2 Assault Buster transformed into Core Fighter, Top Limb, and Bottom Limb.

Underside

Underside

The Mega Beam Rifle can also be split into two and mounted on either side of the Bottom Limb.

Movable sections on the Mega Beam Rifle

Separating the Mega Beam Shield parts

Inside view of the Mega Beam Shield.
The part indicated by the arrow is the pin that locks onto the forearm.

The Mega Beam Cannon and Mega Beam Shield are attached by popping off the red cap on the elbow and plugging them into the hole underneath.

The Assault shoulder armor needs to be deployed outward before attaching —
the attachment method is a bit tricky, so you'll want to practice a few times while following the manual.

Deploying the Assault version's Buster Rifle

Inside view of the Assault armor

V2 Assault Core Fighter / Top Limb / Bottom Limb

Underside

Underside

Underside

Both sides swing out at a wide angle.

You can choose to position the Mega Beam Cannon on either the left or right side.

Inside view of the Buster armor.
It's MG, but there's not much in the way of inner detail molding.

The weapon sections on the skirt and thigh armor can be swapped to an open-style version.
In my case, I only used the open-style when building the V2 Assault Buster configuration, as shown in the example photos.

V2 Buster Core Fighter / Top Limb / Bottom Limb

Underside

Underside

Together with the HGUC V2 Assault Buster Gundam.