FreeCAD’s official GitHub releases now list a 26.3 release candidate alongside recent weekly builds and the older 1.1.4 stable line. That makes it a timely free tool to evaluate alongside Plasticity, especially because you recently wanted to edit an STL whistle without buying Fusion 360.
An STL is a triangle mesh, not a parametric CAD model. Converting it to a shape or solid and exporting STEP generally preserves the facets rather than recovering the original smooth cylinders, sketches and design intent. A simple mechanical part may be workable; a highly detailed organic model may become an unwieldy faceted BREP.
Test with a copy of the whistle. If the output is a huge collection of triangular faces, remodel only the portions you need to edit using the mesh as a reference. A release candidate should not replace a stable production CAD setup until you have tested import, export and performance on your Mac.
Install the Mac candidate in a test environment and try a duplicate STL; compare a faceted solid export with the original rather than assuming you will recover editable design history.