Meaning of the name Didi Chuxing/滴滴出行？
why was it considered acceptable to change 嘀嘀 to 滴滴?
  


This news (嘀嘀打车改名了! 正式更名为滴滴打车) has an explanation about it.


  更名是公司长远战略发展的需要，“滴滴”较“嘀嘀”更具有文化内涵及寓意。【滴】是水滴下落的清脆声，有着滴水成河的凝聚意义。


According to its official 微博, the name was changed for the needs of long-term strategic development, 滴滴 has more cultural connotation and moral than 嘀嘀. 滴 means the clear and melodious sound of water dropping, has the cohesion meaning of "many drops of water make a river".

This news (嘀嘀打车改名为“滴滴”) has another explanation about it.


  依据小桔科技官方的解读，更名为“滴滴打车”，一是取与象声词“嘀嘀”的谐音，二是取“滴水之恩当涌泉相报”之意。


According to the official story, the name was changed because (1) 滴滴 has the same pronunciation with the onomatopoeia 嘀嘀; (2) takes the meaning from 滴水之恩当涌泉相报 (drop the boon of the water, be to flow out spring to report mutually).


  业内分析认为，滴滴改名也是出于商标侵权避嫌考虑。


According to the industry analysis, the name was changed to avoid trademark infringement trouble.

(BTW: 滴滴打车 still had trademark infringement trouble even after changed the name. See 改了名字也侵权 滴滴打车再次成为被告.)


  
  What is the meaning of 嘀嘀 here
  


As the above statement mentioned, 嘀嘀 is 象声词(onomatopoeia), I guess it came from the voice of car horns.