I have used both spellings of the name for the following reasons. During the Boer War & throughout World War 1, the Regiment was officially named The Royal Welsh Fusiliers but was unofficially known as The Royal Welch Fusiliers.. In 1920, the name was officially changed to The Royal Welch Fusiliers.
The Commonwealth War Graves Commission has 10,447 recorded WW1 deaths for the Royal Welsh Fusiliers & 137 for the Royal Welch Fusiliers, all of whom died after the date of the official name change.
,EVANS Griffith William. Second Lieutenant. Royal Air Force & Royal Welsh Fusiliers. Newport Cemetery, Lincoln, UK. 21-04-1918.
As can be seen frm the photograph, this man's grave marker has his rank as 'Flight Lieutenant', not 2nd Lieutenant. I think that he would have been a 2nd Lieutenant whilst serving with the Royal Welsh Fusiliers & later whilst serving with the R.A.F. was promoted to Flight Lieutenant. The CWGC has his rank as 2nd Lieutenant.