Sounds like backwards compatibility is a lock—not that it’s a huge hurdle for today’s machines—since most of the account features involve software and save data. The retro game archives will probably make the jump too, since keeping them locked behind a paid membership is probably more lucrative than the Virtual Console ever was.
Anyone who has ever transferred their account to a new Switch knows how easy it is, for the most part. The biggest chore is redownloading software data, so maybe Nintendo will allow full game data to transfer over via SD card this time.