In Gurbani everywhere it is written GuruDuara but in day to day lives most Sikhs write Gurudwara in English and also the same in Punjabi. I understand that Duar is Punjabi & Dwar is Hindi. I am wondering how did it get changed from Guruduara to Gurudwara in our day to day lives and even in our Punjabi & Gurmat books?
In Punjabi, mostly the spelling used are same as Gurbani for Gurduara i.e. ਗੁਰਦੁਆਰਾ as opposed to ਗੁਰਦਵਾਰਾ but you are correct in saying that in English, the use of "Gurdwara" is more prevalent. This is similar to the use of "Baisakhi" instead of Visakhi (ਵਿਸਾਖੀ). The word "Baisakhi" is the Hindi or the urban use of this word where as ਵਿਸਾਖੀ is the correct Punjabi word for it.
ਸਤਿਗੁਰਬਚਨਕਮਾਵਣੇਸਚਾਏਹੁਵੀਚਾਰੁ॥