There is that famous story where Brian jokingly asked John to play the double bass and a couple of days later finding John with it and had learn't how to play it... so the story goes.... But...?
Almost a year earlier, John, was credited (wiki and other sources) with double bass on Freddie's song Bring Back That Leroy Brown...?
What is the real story behind John on the double bass?? Not sure when exactly Brian came up with that statement, but... wasn't that statement said very early on in their career, possibly not too long after the ANATO release...? or at the very least, 1970's...? Surely he wasn't suffering from memory loss that early on in his life..? Only just a year after John, if he did....? playing double bass on that track of Freddie's with Brian being involved in some part or another for the studio recording or at the very least, vocals and banjo.
It would be nice if the original source would come to light. As for the practicing...a decent bass player like him shouldn't have trouble with that instrument. Maybe with the exception of using the bow.
It's the same instrument more or less. A few changes in technique and in general a bit of acclimatisation with the handling...that's it.
The comment comes from the early 2000's when he answered a question (my question, in fact) about unusual instruments on early albums (he also spoke about the harpsichord Fred played on 'Master-Stroke').
Brian'd joked that perhaps John could try recording the song on upright bass, and he showed up with the instrument a couple of days later.
There's a massive difference between playing a two-bar overdub on 'Leroy Brown' (he could do as many takes as he liked until he got it right, and the whole bit lasted a few seconds) and actually performing on a live backing track (with guitar and percussion) on a three-and-a-half-minute skiffle song.