Naming taboos are common; for example the first month was renamed from 政月 to 正月 and its tone changed from zhèng to zhēng on account of Qin Shi Huang's given name 政. But in this case, 祯 (zhen) and 蒸 (zheng) are not homonyms in modern Chinese.
How were 祯 and 蒸 pronounced during the Song dynasty? Were they homonyms? Or were they considered close enough anyway?