sparq.tools
Got a design made for 1/4" but you're cutting 1/8"? Drop the SVG, confirm what the design was built for, pick your actual stock, and download - slot depths update, panel dimensions stay the same.
You built a box for 3 mm ply and now you want to cut it from 6 mm. Every finger joint, slot, and tab was sized for the old thickness. Thickness swap rescales just that joint geometry so the same design fits the new material, while the overall part dimensions stay the same.
Resizing scales everything. Thickness swap changes only the geometry that depends on material thickness, so part sizes stay correct.
Any sheet material. Set the old and new thickness and it rescales the joints accordingly.