Re the key changes: Many of the banjo players I've played with do re-tune for key changes. I've never seen a fiddler retune because of a key change. There are a few fairly common tunes that use an "alternate tuning," and for these the fiddler would need to re-tune, mostly to get the "double stops" that are part of the tune.

(And if "Silver Bell" is the "Silver Bells" I'm familiar with, the original did not modulate. This is a "sop" to the guitar players who find it easier to swap the "low line" into a different key - but one might question whether it should be left as a fiddle tune, in one key.)

The essential difference I see is that "Blue Grass" is the adapting of old time tunes to radio performance. In the old-time style, the players all played together, and people danced and sang along with them. When converted for radio performance, most people just sit and listen. In order to "put on the show" in a way that will keep the audience, bluegrass switched to an emphasis on "solo" performance, and on "professional virtuosity."

In bluegrass, when one instrument plays lead, the other instruments back off to a really boring "chop, chop, chunk, chunk" so that the virtuosity of the lead can "star." And for a "radio grade" performance, the lead should display virtuosity of the "we're professsional, don't try this at home" grade.

In old-time style, the other instruments would be more likely to play harmony or counterpoint to the lead line, hopefully following what the lead does - or at least not drowning out the lead.

A somewhat unfortunate "growth" from the virtuosity of the best bluegrass players is that "wannabe's" hear all those notes from the virtuoso player and think that playing faster means playing better. Sadly, that means that many very nice tunes get hacked up to leave out the hard notes, just so they can be played faster, get moved to keys that are easier for the guitar (a leading offender), and otherwise get transmogrified into "bluegrass" versions. Even Bill Monroe objected that "most people try to play too fast."

It ain't the speed. It's the drive.

Not a complete differentiation, but bluegrass is for a "listening and non-participating" audience. Old time style generally assumes audience participation.

Bluegrass is a sequence of solos. Old time style is (often) harmony and counterpoint.

John