UV Map issue (Shortening a Full Body Dress)

Edit: Sorry, didn't update the page again
Hi.
1). Before combining the nude body's and clothing's meshes, you need to delete the parts of the former that are covered by the later.

To do this:
- switch to orthographic ( 5 key ) front ( 1 key ) view to have an easier time selecting things;
- select the nude bottom, switch to Edit mode ( TAB ), switch to Vertex select ( 1 ), ensure Limit selection to visible ( 2 ) is disabled, press B to activate box selection and select the covered part of the body like shown on the pic ( 3 );
- press Delete key and choose Faces ( 4 ) to delete. Make sure to leave a row of polygons ( 5 ) or there will be holes in the process of walking;
2). Generally, outfits are split into a number of meshes (s4studio_mesh_) to interact with other CAS items. Each has a cut number assigned which is what allows s4s to recognize which meshgroups of the original blend should be replaced with which of the one you're importing ( more about it here, please read it until you get it or you will often run into the problem of missing parts in s4s ).
However, sometimes they are split in ways that go against this logic. And this is the case with the outfit you're using as a base.
You know how when you're choosing high boots, the pants the sim is wearing "roll up"? The lower part of the pants disappears to not poke out from the boots. So by this logic, like most similar outfits, the outfit you're using as a base should be split into 3 to properly interact with boots of different length. Yet it is split in 6 with some random polygons separated.
So, to have an easier time editing everything, you should combine them and clone a different BASE GAME outfit to import your new blend.

- start by selecting and deleting 2 rows ( 1 ) of polygons from the bottom because they overlap with s4studio_mesh_5;
- then select 3 rows of polygons and separate them by pressing P - Selection ( 2 );
- switch to Object mode (TAB), Shift-select all s4studio_mesh_ BUT s4studio_mesh_1.002 and s4studio_mesh_5, join everything with Ctrl J;
- change s4studio_mesh_6's cut number ( 3 ) to 0002, s4studio_mesh_5's to 0000, s4studio_mesh_1.002's to 0001;

- select s4studio_mesh_6, switch to Edit mode, holding Ctrl, lasso-select both legs ( 1 ), Select - Linked ( 2-3 ) to select them fully. Then choose Select - Inverse ( 4 );
- set DiffuseMap as a background image ( 5 );
- in UV editor press A to deselect, press B and select the skirt like in the pic ( 6 );
- press S to scale, Y to scale only vertically, type in .45 to scale it down, LMB click to finish. Press G to move, Y to move only vertically, type in 110;
- in 3d window press A twice. In Uv Editor select the skirt again, using S and G adjust the uvs until they look like this ( 7 );
- holding Ctrl, lasso-select both sides like this ( 1 ). Select - Select linked. Press G to move, Z to move vertically in the 3d view, lower it a bit until it looks like this ( 2 );
- repeat the same like this ( 3 - 4 );
- press Shift H to isolate the selection, select this vertex loop ( 5 ) and remove doubles ( 6 ) in Tools;
- in UV editor press A and go to UVs - Export UV Layout. You will use it as a guide to adjust the length of the skirt's texture in the graphic editor of your choice;
- press Alt H to unhide everything.
Don't forget to do the same for the uv_1 map and to re-paint the weights for the skirt.
Start a new project from this dress ( 7 ) and import your blend there.