I didn't link the diagram, but the changes from major to minor are only in how those chords are labeled, not what number they are.
Just to give a quick primer on reading roman numeral chords - capital is major, lower case is minor. If there's a degree sign (°) next to the lower case, it's a diminished chord. If there's a plus sign (+) next to the upper case, it's an augmented chord.
In Major, the diatonic chords are I, ii, iii, IV, V, vi, vii°, I. In Natural Minor, the diatonic chords are i, ii°, III, iv, v, VI, VII, i.
I also try to avoid that sort of chord flow chart because, as the label on it says, it's a "simple" map and adheres to a very rigid order that isn't always followed.
For beginners, I actually prefer something my harmony teacher gave us that he called the function triangle -
It works similarly to the flow chart, but imo feels less restrictive in your options while adhering to the same concepts. All you need to do is follow 3 rules.
1. Start on I. You may jump from I to any chord, or jump from any chord on one side to any chord on the same side (you can jump between ii, IV, and vi as much as you want, and between iii, V, and vii° as much as you want - unless you land on V).
2. You may move clockwise by one side (From I to ii/IV/vi, from ii/IV/vi to iii/V/vii°, or from iii/V/vii° to I), but you cannot (until you learn how) move backwards.
3. When you get to V, you have to go to I.
Obviously there are exceptions and variations on this, but if you follow that chart you'll never get lost. Once you get comfortable with it, you can experiment with things like tricking people by going from V to vi instead of I!