Rubricarius is correct, except one small point. The tweak started in 1960, not 1961. That was immaterial in practice, since the Immaculate Conception in 1960 was on a Thursday. Dec 8 was next a Sunday in 1963. So in 1963, the Second Sunday of Advent was in fact the Immaculate Conception with a commemoration of the Sunday. Were you to look up an ordo for the year 1963, it would show this.
The 1960 Rubrics are very clear. The Immaculate Conception outranks the Second Sunday of Advent. In no. 91 of this set of rubrics, this could not be more clear.
For Rubricarius to call the 1960 rubrics a "tweak" is inaccurate. It isn't really a tweak except in effect.
In 1960 a whole new system of liturgical classification was introduced. It tried to keep the effects of the older system, but make the classifications simpler and more clear. You cannot read the 1960 rubrics in the light of what came before except that it was an effort to have the same effect, without the same complications.
Previously you had Ferias, Major Ferias, Simple feasts, Semi-Doubles, Doubles, Secondary Greater Doubles, Primary Greater Doubles, Secondary Doubles of the Second Class, Primary Doubles of the Second Class, Secondary Doubles of the First Class, Primary Doubles of the First Class, major Sundays of the Second Class, and Major Sundays of the First Class. The 1956 changes merely simplified this older system getting rid of many classifications. The 1960 system replaced it with four numbered classes of liturgical days and a table of precedence when there was a conflict among days of the same class.
The equivalence is that :
1st Class = Doubles of the First Class + Major Sundays of the First and Second Class (except Septuagesima, Sexagesima and Quinquagesima) + Privileged Major Ferias + Triduum + All Souls + Octave Days of Easter and Pentecost
2nd Class = Doubles of the Second Class + a few Greater Doubles (the Holy Family, the Exaltation of the Holy Cross, and feasts of secondary local patrons) + Non privileged Major Ferias + Sundays not included above + Ferias of Advent from Dec 17-23
3rd Class = Remaining Greater doubles and Semi-Doubles + Ferias of Lent + Ferias of Advent up through Dec 16 + Octave days of Christmas
4th Class = Simple feasts (reduced to commemorations), BVM on Saturdays, Ferials except those of note
In 1960 you now had days of the 1st, 2nd, 3rd and 4th class. The higher class always took precendece. When two days of the same class happened to concur or occur, one consulted the table of precedence (no. 91 in the 1960 Code of Rubrics). Generic charts for occurance and concurrence were created, but these are not 100% reliable. If you look at these charts alone, for instance, one finds that the Immaculate Conception would be transferred. The actual rubrics, however, are clear that this is not the case.
Today, Dec 2 is St Bibiana, a 3rd class feast. It is also a day in Advent before Dec 17, so it is a 3rd class feria. Which is celebrated? The table of precedence puts Advent ferias at no. 25, and 3rd class universal feasts at no. 24. The feast is higher, so it takes precedence, and the feria would be omitted, except it is given a privilege so is commemorated.
In fact the reason that during Advent you celebrate 3rd class feasts, but not during Lent is due to this table. 3rd class feasts are at no. 24. Ferias of Lent are at no. 22. Ferias of Advent through Dec 16 are at no. 25. Thus a 3rd class feast loses out to the Lenten feria, but takes precedence over the Advent feria, even though all of these are 3rd class.
Back in September Ember Saturday (2nd Class) fell on Sept 21, which is the feast of St Matthew (2nd Class). St Matthew is at no. 16 on the list. The Ember Day is at no. 18. St Matthew takes the day.
The same happens on Sunday. You have the occurrence of both the Second Sunday of Advent (1st class) and Immaculate Conception (1st class). The Sundays of Advent and Lent sit at no. 6 in the table. The Immaculate Conception (and Annunciation) sit at no. 4, so those feasts are higher and take precedence.
The "rubrics" of the Missal were not changing up through the Novus Ordo, as you suggest. The texts of the missal and interlinear instructions did in 1964 and again in 1967. The rubrics that establish the calendar, commemorations, etc. were exactly the same as in July 1960.
It is worth noting that the Code of Rubrics was attached to Novum Rubricarum, and was not actually part of the document, so if you're looking in NR (and it's a few paragraphs long) or Rubricarum instructum for these, you're not seeing the actual rubrics. That is a code which is 530 point long.