Upgrading stroller from single to double

[mirage0306]

Hello Disney families! Anyone happen to know how it works if you rent a single stroller from Disney and then decide you would like the double? Will they allow you to pay the difference, or would they treat it as a totally different transaction and charge us the full amount?

Flame away, I know how old my kids are, but I also know I do not want to handle bickering and unhappiness over a stupid stroller in Disney so I'm trying to plan ahead. Thanks!