high resolution height map without loosing data

This is a tricky one.

If what you want is to "add" more terrain, I would do something like this:

     void ChangeTerrainResolution (int newResolution)
     {
         Terrain t = Terrain.activeTerrain;
         TerrainData d = t.terrainData;
         float[,] heights = d.GetHeights(0, 0, d.heightmapWidth, d.heightmapHeight);
         
         int previousResolution = d.heightmapResolution;
         d.heightmapResolution = newResolution;
         float[,] newHeights = new float[newResolution,newResolution];
         for(int x = 0; x < previousResolution; x++)
         {
             for(int y = 0; y < previousResolution; y++)
             {
                 newHeights[x,y] = heights[x,y];
             }
         }
         d.SetHeights(0, 0, newHeights);
     }

If you want to actually be able to "fill in the gaps", you will need to interpolate new height values and it will be somewhat more complex.

Yeah I want to "fill the gaps" and that's actually where I'm struggling