I noticed that pattern during Season 10 as well, but the "new becoming old" wasn't as extensive as in Seasons 3 through 8.
During Seasons 11 through at least 14, there were clusters of episodes with the "new becoming old" letters format, sometimes skipping a number due to the so-called formula which is supposedly used to determine the number of the day. It implies that during Seasons 3 through 8, episodes were aired in production order, while Seasons 9 through at least 14 likely did have the "new becoming old" letter format in production order, but the episodes were likely no longer aired in production order. The snow arc episodes from Season 13 did not have the "new replacing old" format, implying they may not have been taped consecutively, but sporadically.
It's also observed that a March 1983 episode (episode 1790, to be exact) was likely taped way back in November 1982, and not aired until March 1983, since Mr. Hooper appeared in the first street scene. It may very well have been Mr. Hooper's final appearance on the show, or one of his last appearances. The featured letters were D and K, and "D" had lesser airtime than "K" did. The number of the day was 8, but to the best of my knowledge, if the episode followed the "new replacing old" format in production order, episode 1714 may have been the next taped episode (aired months before Episode 1790) since "K" had the lesser airtime, and the new letter was "Q", and had more airtime, and the number of the day was 9. Only two episodes that season had the letter K appear with the number 9, and initially I had thought episode 1789 was the only one (which was paired with "H").
During the Camp Echo Rock week of episodes (first week of Season 14), footage was taped at the camp from August 16 to 20, 1982, and in the studio during October 7 and 8, 1982. The first episode of that week had the letter D as the lone letter, and two pairs of letters each had the letters Y and V (new, then old, but non-consecutively aired). However, the letter C (which had less airtime) was used in the earlier V episode. It makes me wonder if they used C because the letter D was used in two consecutive episodes, if you include the Season 13 finale.
I do know that, by Season 9, there have been episodes with letter pairs in which both letters sounded or looked almost similar (B vs. P, M vs. N, X vs. Y, etc.), as was mentioned by another user earlier in this thread. However, I did notice that H and I were common pairings during the late 70s and early 80s, and I don't see any similarities in sound nor appearance. The first many weeks of Seasons 11 and 16 had episodes with pairs of consecutive letters, which made these seasons boring at first, as I am not a fan of pairs of consecutive letters, and Season 16 relied on consecutive letter pairings for many, many weeks (not including the four weeks of reruns), and because of that, I had lost interest in the show by February 1985.
I did an observation with the number of the day for Season 13, and I noticed the number 5 was used the least number of episodes that season.