Which pair of reindeer did Rudolph replace when he saved Christmas in the 1964 animated classic?

Well I can't say I have confirmed this, but this Wikia on Christmas specials claims to have:

There are only six reindeer in addition to Rudolph, instead of the usual eight. It has been confirmed that there are only six reindeer because of the time that the creators had to make them, and because they were rather expensive to build.

Santa: "Well, let's hope so, if he wants to make the sleigh team someday. (to Rudolph) You see, little fellow, every year I shine up my jingle bells... for eight lucky reindeer." Transcript from https://rudolphtherednosedreindeer.fandom.com/wiki/Rudolph_The_Red-Nosed_Reindeer/Transcript

It seems to me the reindeer in the song are only the famous Generation 1 reindeer. The film has Donner in a coaching role for the next generation competing for this role. He is not hitched to the sleigh when Rudolph leads the team at the end of the film.

So asking why an 8 or 9 Reindeer team wasn't used in an emergency would be valid. However, there is no reason to assume any pair you can name were replaced. The team for whatever reason had only 6 on the team and one can assume all were younger than Donner and Comet's generation. Note also the other answer here mentioning Scratcher resenting being cut from the team - establishing further that year was a team of reindeer you haven't heard of.