There was a lot of changes with the original 737 and the Max.
First of all the original 737 had a wingspan of 93 feet the 100-500 model had this wingspan
the 737NG has a wingspan of 117.5 feet
and the 737 Max has a wingspan of 118 feet, so it has more lift.
Another change would be the engine, the Boeing 737 100-200 had a Pratt & Whitney JT8D,
the 737 300-500 had a CFM International 56-3 series
and the 737 600-900 had a CFM International CFM56-7 series
and the 737Max has a CFM International LEAP-1B
which is the most efficient engine out of the whole 737 series.
Another major change is the winglet.
The winglet started to being put on the 737NG series and it increases efficiency,and lift
but the 737 Max wing has a split tip winglet which increases efficiency and lift by 1% over the original winglet and also the wing is raked like a Boeing 787 to reduce wing tip vortex.
Another major change is lower fuel costs than the original 737 and lower operation costs.
Another change would be the tail cone which is more efficient.
Second to last major change would be touchscreen screens on the back of the seats which modern airplanes like the 787 and the A350 have.
The last major change is the sky interior.