Keep UV islands the same after zbrush remesh

Hi. You can preserve polygroups and then transfer uv_0 the same way you transfer uv_1.
That said, I’d advise you to just unwrap the mesh in blender, really. You just need to put seams in places where they were originally (or close).
The uvs you get from MD are stretched or shrunk (unless you tailor the garment exactly like real tailors), so it poses a problem with projecting textures on them (if you want to use patterns or graphics).
If the seams are in the same places they exist on the real garment, you’ll get nice uvs after unwrapping

People say that if you will mark seams on your item and on the reference, it will take that into account. It never worked for me, even when i split them, but you can try.

Another way is to select all border edges, assign a new vertex group to them, and transfer uvs with the default Face corner data for this group only. It doesnt work in most cases, though, lol.

And the fail proof way is to separate each pattern of your retopologized item and of the reference, then transfer uvs from the front(reference) to the front(retopologized), back to back, etc. Note that sleeves would have to be split into 2 (front_sleeves and back_sleeves)

- import the original garment from md to blender. If you exported it from MD as unweld, switch to Edit mode, select everything with A, P - By loose parts, then give each part reasonable names (r_front, r_back, r_sleeve_f, r_sleeves_b, etc). If you exported it as well, select its uvs map, select everything with A, UVs - Seams from islands, select one edge marked as seam, Select - Select similar - Seam, Mesh - Edges - Edge split, select everything, P - By loose parts, give names;
- import your mesh from zbrush. If you used polygroups, it will come split into multiple meshes, so just rename them(new_front, new_back, etc);
- transfer uvs from r_front to new_front, r_back to new_back, etc

You need to separate not the left sleeve from the right sleeve, but the front on both sleeves and the back.
In uv editor select half of the uv island of the sleeve and that will be the front (or the back), do the same on another sleeve, separate the selection, name it, name the remaining one. Do the same with the retopologized mesh

Select that edge loop and W - align x, then W - remove doubles with 0,0001(or more). Then in uv editor press Q, open the panel to the right (try N maybe it will open it), then change Grab to Relax and enable something like Pin borders, run along that straightened edge loop