到 vs 至, what is the key difference?
Both 到 and 至 can function as "preposition" (to) or "result/ degree complement" (until; to the point of). The difference is 到 is more colloquial and 至 is more literary

Example:

戰至一兵一卒 (fight to the last man) is a very literary phrase, in colloquial form, this phrase would become 戰鬥到剩下最後一個士兵. 

It is not recommended to replace  "至" with "到" in literary phrases like 戰至一兵一卒, 延至凌晨 (delay until midnight)  and specially not in idioms like 至死不渝  ( you cannot say 到死不渝) 

Other differences:


Both 至 and 到 has the meaning of "arrive" and the difference is still literary and colloquial. 
至 has the meaning of "ultimate" or "utmost" that 到 doesn't has. e.g. 至高 cannot be changed to 到高
到 can function as a result complement that indicate the verb is successful, e.g. 學到 cannot be changed to 學至