Are chords the same building blocks as notes in a song? Like do every song consist of certain chord progressions? You can reharmonize a song by changing the chord progression can't you? But you can't change the notes to much of a song without changing the song.
Does every song consist of certain chord progressions?
There's a lot to that question. First of all, not all songs even have chords! And it is true that some chord progressions can become famous or recognizable—Wagner wrote a certain chord that moved to the next chord in a certain way, in his opera Tristan & Isolde, and it became known as the "Tristan chord." But we recognize individual melodies much more easily than chord progressions. Millions of songs use the progression "I IV V I," but we tell them apart by other things like melody.
You can reharmonize a song by changing the chord progression can't you?
Yes! "Reharmonizing" a tune, giving it a new set of chords, is a fun practice that has been going on for centuries. Bach filled whole books with "harmonizations," sometimes giving the same tune dozens of different chord progressions. And we still consider it "the same tune" by recognizing the melody. Consider Schnittke's Stille Nacht:
The familiar tune of the Christmas song runs throughout the piece, but the harmonies become increasingly unfamiliar and unsettling.
But you can't change the notes to much of a song without changing the song.
There's a lot to say about that too. Change a note or two, and you're just ornamenting. You could change so much that you've created a "variation" on the tune—but what still makes it identifiable as a "variation"? At what point have you created a whole new song? Copyright lawsuits have to consider this. Is George Harrison's "My Sweet Lord" different enough from The Chiffons' "He's So Fine"?
Every song you hear that's written and recorded will initially have the same chords (and same melody) as original, but everything can change after that. If that's what you're asking.
If you're considering the diatonic notes and chords, that's quite a different issue.
But It's not unusual to change the chords that accompany the melody of a particular song - jazz players will do that almost as a matter of course! Although they won't change the structure so much that the song is unrecognisable. At that point, it's a new song!
So, yes, the chord structure of an existing song can be, and may be, changed, as long as the changed chords still fit with the melody, otherwise, it's a pretty pointless exercise. At that point, though, the song remains the same, as it still retains its title and melody.
You've stumbled upon the DNA of music. Yes you can re-harmonize any piece of music as you see fit but changing the song's chord progression is a little more tricky, especially if you are playing with other musicians. Jazz musicians re-harmonize all the time but most wouldn't change a progression unless it fit the regole or rules. As a soloist you may do whatever you like. I'd still steer clear of changing a progression and settle for substitutions.
There is a 17th century method of improvisation called Partimento where the first lesson is harmonizing a scale. Every basic chord consists of a note's 1, 3, 5. A C scale is CDEFGABC or 12345678. Start on C and play IT'S 135 which would be CEG. That is a C major chord. Then start on 2 which is a D and play its 135 which would be DFA or D minor. The rest of the chords are E minor, F major, G major, A minor B diminished and C. This is called the rule of the octave and you need to learn those in all keys. Most every song can neatly fit into this rule. If you are harmonizing something like a chorale in the style of Bach, you can look at a melody and look for a chord which has that note. If your melody note is E in the key of C the chords you would have at your disposal would be C, E minor, A minor. You can also experiment with substituting diminished chords, other minor chords, chords with sixes or sevenths in them and the most exciting . . . MODES.
If you were to re-harmonize a song such as ODE TO JOY, play it once in it's hymn setting to get a feel for the harmonies they try it with the rule of the octave - Playing it in the key of C try these chords following the octave. Each chord changes on the bar: C G/B F/A G Fmaj7 C/E D minor G7 C. That is a basic Ionian rule of octave substitution. You can do that to most every song and it can sometimes work with chromatics. You can also do this with all the modes but they all would require too much work for me to write out here.
Actually, Mixolydian would be easy. Mixo has a flat 7 so play the C scale but with the 7th flatted which would have a Bb instead of a B. Taking Ode to Joy again, play a C chord then a Bb, C, Bb, C, Bb etcetera.
There are a couple hundred regole in the method and the options and possibilities are mind boggling. As I mentioned, diminished chords both on the note or half a step from the note works. Tritones work nicely. Take the song BLUE MOON in the key of C, the chords are C Am Dm G7. Before each chord play it's tritone. For this example each chord is on the quarter note: C Bb A7 Eb7 Dm Ab G7 Db C.
There, that is 15 years of study for you.