Ramadan & Ramzan are both correct. Neither is 'pure' or 'original'
Every year in the month preceding Eid ul-Fitr a familiar debate resurfaces: do we call this month Ramzan or Ramadan? This is quickly followed by another question: which pronunciation was the correct one at the time when the Quran was written?
As it happens, the answer is: 'neither of the above.' We are fortunate to have an exact description of how the modern Arabic sound
ض(Daad/zuad) was pronounced back in the eighth century by the linguist, Sibawayh, whose grasp of articulatory phonetics would have warmed Panini's heart.
ضas an 'emphatic lateral fricative', where the airstream is not obstructed as it would have to be for a d-type sound. Fricatives are sounds like 's', or 'h', or 'f', where there is a continuous flow of air with a hissing sound.
ضin those days was a grooved fricative, which means that it sounded similar to the 'zh' sound in Russian, or the sound in the English words 'pleasure' or 'measure', except that it was also ejective, or expelled with a bit more air pressure. Ramzh'an.
It is easy to see how the early Persians would have heard this grooved fricative, 'zh', as the more familiar flat fricative 'z', and as Persian was the language of culture and erudition not just in Iran but through all of Central Asia, the Persian way of pronouncing
ضspread. The Mughals brought it to India, where the entire territory under Urdu's footprint has for centuries been saying 'Ramzan', just as the Turks, moving westward into Anatolia, took with them the Persian words 'Ramzan' and 'namaz', along with a number of other words we know from Urdu. In that sense, looking at the geographical spread of the word 'Ramzan' is like seeing back through time into the medieval history of Iran, Central Asia, Turkey and India.
ضturn into the pharyngealised d-like sound we are now being asked to switch to by saying 'Ramadan'? At a guess, I would say that it has to do with it being 'ejective', or expelled more forcibly than other consonants. Ejectives do often evolve into affricates, with an initial 'd' being added to build up a bit more pressure. Think of the difference between 'ch' and 'sh': the release of the 'ch' is more forceful, as it has an initial 't' to build up pressure before releasing the 'sh'.
In most Gulf Arabic,
ضvery quickly merged with the letter
ظ(DHa/zoe), and both of these became pharyngeal d-sounds, or 'd' sounds with a cavernous muted echo. Arabic, like any other living thing, was growing and evolving as it lived through changing times. Modern spoken Arabic is quite different from the language of the Quran, and while the words and grammar of the early days have been preserved because of the written text, modern touches have slipped in via the pronunciation.
'Ramadan', then, is Arabic, but it is not 8th century Arabic, and thus it cannot lay any special claim to being the pronunciation used when the Quran was first written. In purely linguistic terms there isn't any reason to favor 'Ramadan' over 'Ramzan'.
Modern linguistics is not concerned with prescribing rules of pronunciation and grammar, but instead celebrates the way words and entire languages have grown into new ecosystems. Loanwords are expected to reflect the speech habits of the communities that borrow them, so there is no surprise in Ramzhan becoming Ramzan and even, in Bihari villages, Ramjan and, in the Caribbean, Ramjon. That shows the mixing of two parental streams, as new concepts and influences enter the lives of old communities.
What is interesting about the appearance of the word Ramadan into the fray is that it too is a bit of history happening in our lifetimes, before our eyes. The entry of 'Ramadan', and 'Allah hafiz' as a replacement for the more Persian 'Ramzan' and 'Khuda hafiz' reflects the growing influence Saudi money has in this region, and the way it is mounting a challenge to what it sees as a competing centre of power: Iran. What in the old days was done by military conquest is now increasingly done by economic means. It is just possible that those pushing modern Saudi pronunciation sincerely do believe that the way they pronounce Arabic now is identical to how the people living in Makkah spoke back in the 8th century, simply because both these varieties have the same name: Arabic. Some of them may genuinely believe that what they are offering us is the 'pure' original option.
Purification is a very misunderstood process. When we set out to purify something we have to introduce a new element, maybe a chemical, to separate out what we don't want so that we can remove it easily. In the process we have added something new, something we are not conscious of as alien. But we are happy, because the final result looks the way we want it to.
In the end your decision to say Ramzan or Ramadan (or Ramjan, or Ramjon) reflects your political choice. Culture is not carved in stone, and loyalties keep changing, as people move towards whatever makes them feel more secure and more empowered. As a linguist I don't care: I just sit and watch, knowing all of this has happened before, and will happen again, and that change, and the pull and tug of conflict, are part of the essence of life.