The tonal steps define the scale

What defines the scale is the combination of Intervals.

If you change those then the scale changes.

If you change the intervals the scale changes.

But you can change the modal steps of the scale without changing the scale itself.

I don`t understand what you mean when you say "tonal steps" and "modal steps".
Half-steps are "semi-tones" or from one fret to the next fret on the guitar.
Whole steps are "tones" or two frets apart.
And that is the same for the main scale or its modes.

But if you flatten the 3rd (a modal step,) then the scale does change to a minor scale

If you flatten the 3rd of the Major Scale, you get minor third. Becouse the major scale has a major third; and everytime you flatten any major interval you get a minor interval.

The biggest mistake people make, is to play modes and keep thinking in the major scale.
You see, every mode is an individual Key in itself. Just like when you play a "natural minor scale" which is the sixth mode of the Major Scale (Aeolian Mode).You know that you are playing a minor key.
In the same way a Dorian Mode is a minor key, and so is the Phygian Mode,
Here are the tonalities of each mode:
Ionian........Major
Dorian........Minor
Phrygian.....Minor
Lydian........Major
Mixolydian...Major
Aeolian.......Minor
Locrian.......Diminished