Why wouldn't the 20/29 be capable of conversion if the 21/30 are? The frames are the same, they have different uppers on them. /quote]
I believe you have it backwards - 9x25mm barrels are available for the G20/G29, but not the G21/G30. Since the .45ACP slides have a wider breachface and thus require a wider barrel hood, one cannot simply drop in a barrel intended for the 10mm Glocks (the 10mm conversion barrels for the G21/G30 accommodate this difference).
It would seem that this would be an easy issue to overcome for a barrel manufacturer, but I suspect the demand for such a product is not exactly overwhelming. </div></div>
That's exactly what I thought, it seemed that the advice higher up was that the 10mm guns couldn't convert but the 45's could...
And to convert my 20 and old 29 it was just a barrel swap.