The “first marriage” would have some presumption of validity.
Reasons which justify choosing a subsequent wife (we’ll ignore the chance that it could be a woman with multiple husbands) have included, historically, the first wife’s opposition to the Christian faith of the convert and his practice of the faith, the absence of the wife (abducted or whatever the reason might be), the man’s inability to remember which woman he married first. The current canon simply says he can choose a subsequent wife “if it is hard to remain” with the first one. The “hardship” should be, I would think, something objectively difficult.
Whichever woman he chooses, he has to marry after his conversion. This would dissolve any valid marriage bond he might have.
Dan