From Dylans famous song of course. here is my question : Why the a in “a-changing”, what does it mean, or is it just for rythm purposes?
It’s just an affectation. Lots of dialects say things like “I’m gonna go a-fishin’.”
If something is big, it uses the “a,” as in “there’s a hurricane a-coming.”
At least according to Sofia Petrillo of the Golden Girls
That can’t be it, because Froggy went a-courtin’.
Maybe he had a Big Big Love.
That’s more to avoid the trochee.
The big is the courting part. Froggy just wasn’t looking to get lucky, he was looking for a wife, which is big 
I hear it occasionally here in Texas, usually in the phrase “come a-running.” Ex. “Just tell him that supper’s ready and he’ll come a-running.”
I can tell a storm’s a-comin’ when my corn’s a-twitchin’.
It’s a fairly old construction - think of “Sumer is icumen in” (“Summer is a-coming in”).
From this commentary on “Sumer”:
So “a-changing” likewise implies that change had already occurred (which was true, really), rather than in progress or in the future.
Woody Guthrie, in lyrics of Mister Charlie Lindberg:
“Then they had a meetin’, and all the Firsters come,
Come on a-walkin’, they come on a-runnin’,”
And in Dylans case, the ‘a-changin’ does seem to fit the scan better.
It’s a worn-down unstressed variation of the preposition on, from Old English an ‘on’. To be “on” a verb was an Old English way to express progressive tenses. The loss of final -n when unstressed parallels the same thing happening to the word’s homophone an originally meaning ‘one’, but which became a when used as the indefinite article.
The AHD also notes the survival of the a- prefix in the dialect of Smith Island in the Chesapeake Bay.
The preposition on, OE an, derives from Proto-Indo-European *an- meaning ‘on’. Other words tracing back to this root include Greek ana ‘up’ (as in analysis, Anabasis, or Anatolia) and Russian na ‘on, in, upon, into’, as in Rostov na Donu.
And he did ride?
Uh-huh!
(Weapons?)
This. It is a archaic leftover from the German pluperfect verb tense, which adds “ge” to the beginning of most verbs. This is the reason it is almost always used with a helping verb, most often “to be” or “to have” or their congugations…“are” in the Dillon lyric. (“to be”, congugated to match “they”)
As with most German leftovers, like placing prepositions at the end of sentences, it is seen as low class, or “Folksy” if you want to put a positive spin on it, which is probably exactly what folk singer Dillon was aiming for.
Dylan!
As in Dylan Thomas
(Kevbo – that man ain’t got no culture 
Krambo!!
I dropped my harmonica, Albert.
ETA: What is a krambo, or crambone anyway?