Blaster’s chunky triangle motifs are the feature of the pattern, but they also make it a little tricky to modify and customize. The pattern has triangles at the base of all of the eyelet columns, and for some sizes, there are additional triangles at the sides, so they fully wrap around the waist with no gaps of plain stockinette. If you adjust the waist shaping for better fit or convert the cropped cardigan to full length, you may run into trouble with placing the triangles.
If you want gap-free triangles, you need to have stitches in an increment of 18 plus 3 edging stitches, and you’ll need to keep in mind how the columns of eyelets will flow. Your safest best for a gap-free set of triangles is to add sets of 36, one extra triangle at each side. Now that might be manageable if you’re adding length and need hip shaping, but it’s a lot of extra fabric if you just want a looser waist. It’s also not much help if you want a smaller waist for your cardigan!
If you’d like to add or remove from the waist without much fuss, you’ll need to add sections of stockinette in between triangles at the sides. Use the eyelet columns as guidance for the triangles that do fit, and fill in the rest with plain stockinette. If you’re feeling more ambitious, you can fit in shorter, smaller triangles in your stockinette sections. You’ll want to start those after you’re partially through the full-size triangles, so they sit nicely next to their neighbors on the final rows before the ribbing.