When using geographical names romanized according to the previous 1967 version of the official Thai romanization system, it is essential to know that earlier no distinction was made between the vowels u and ue (u was used in both cases, [...]) and some diphthongs were romanized differently (iu for character 48 /now io/, ieo for character 53 /now iao/ in the table mentioned).I wasn't aware before that the romanization system, abbreviated RTGS for "Royal Thai General System of Transcription" in fact just dated from the year 2000, when a bigger revision was made. Now I checked it I even found the official announcement of the RTGS in the Royal Gazette (ประกาศสำนักนายกรัฐมนตรี เรื่อง หลักเกณฑ์การถอดอักษรไทยเป็นอักษรโรมันแบบถ่ายเสียง).
This explains why in my first trips to Thailand I still saw the old airport spelled "Don Muang" everywhere, and only recently the street signs pointing towards that district have changed to "Don Mueang". I thought the policy to have street sign follow RTGS was enforced in the last decade, but now I know it was only the modification which made me notice that policy. But as you can see in this photo of the railway station Don Mueang (from the ThaiTransit blog) the old spelling still has survived.