I can copy a reply given on manxforums (www.manxforums.com/forums/index.php?showtopic=19450&hl=ewan%20juan&st=30) by freggyragh (a pseudonym) which summarises it well -

Juan, Yuan & Ewan are the same.

Juan - Nominative / Accusative
Yuan - Vocative / Dative / Genitive / Possessive
Ewan - alternative spelling of Yuan - also common alternative spelling in Scotland , eg McEwan - the son of Juan.

John / Ewan / Ian - Common English translations for Juan found in old official documents. (It was common to have two names in the past, a Manx one and an English one - eg Neddy Beg Hom Ruy - Edward Faragher, Illiam Dhone - William Christian.)

Celtic nouns, including proper names, may lenite inititial constonants and broaden or slenderise vowels according to case.
The sound of 'j' lenites to 'y'. The letter J did not exist in Old Gaelic - most of the 'j' sounds in Manx are actually lenited 'd's -- and are written as such in Irish and Scottish. Because the Latin form was Ioannes, and Old Gaelic would not have had a 'J' sound in a name's initial position, the older form was Ean.

There are plenty of instances of Ean / Eoin /Ioan in Old Gaelic. Things changed with the arrival of the Normans, but the form Ean is still used in Manx when referring to Biblical characters, or in placenames connected with a saint - usually in the form Eoin (pronounced 'Owen') - meaning 'of (Saint) John'.

The Normans followed a trend of make Latin 'i' sounds into 'j' sounds - eg Iudea - Judea, iuvenali - juvenille, iudex - judge. The Norman name Jehan was their attempt at the Latin Ioannes. Pronounced with the stress on the first syllable (as Norman French probably did), you get something quite close to Juan.

- The Irish liked the name but were too conservative in their grammar to allow a name to begin with J - so in Ireland Jehan became Sean, The English liked the name but shortened it John, the French liked it but shortened it to Jean. The Manx just added a slight vowel shift to the first syllable to get Juan - and of course the rest of Europe with their Johanns and Juans weren't under the Normans so didn't take up the 'j' sounds at all. By this reckoning I would say Juan has been a Manx name for nine hundred years or more and is as Manx as Sean is Irish, Jean is French and John is English.

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I could add that the Ewan was often written Euan in the records (Latin had no W) - this was then transcribed as Evan (possibly confusion with Welsh name) - because of this easy confusion I suspect many Manx Ewans who settled abroad moved name to be Hugh which sounded more similar - note also that the IGI seemed to baulk at transcribing Hu from the records (often a blank is left where the origin is very clear)