I am really happy that R/G/B/Y are finally coming to Virtual Console, first of all. I didn't think they'd ever make it to there because of them relying on the Link Cable. Glad they found a way to 'virtualize' that feature.
As for any significant changes, I expect that they might close the Game Corner (making it little more than a gate to the Team Rocket hideout), or modify it to the point where playing its games doesn't really involve gambling. I also expect that the Game Boy Printer features will be disabled (on Yellow) as well (as that's what was done to the VC release of Super Mario Bros. Deluxe).
The Old Man MissingNo glitch might end up going as well, if the games are modified in the ways mentioned above. Then again, with the way Generation I is, changing some aspects of it might open up a whole new set of glitches.
I do believe, though, that there's a small chance that the Generation I classics will be re-released with very little changes at all (apart from the support needed to trade via infrared, which might actually be done to the Virtual Console backend itself.) It would be a bit of a precedent if the MissingNo Old Man glitch remains intact, given that the VC re-release of the original Super Mario Bros. retains the infamous "Minus World" and "Stuck underwater" glitches.
I also would hope that we'll see Generation II's entries (Gold, Silver, and Crystal) re-released in the same fashion at some point in the future following Gen. I as well.