What is the ruling on changing one's intention from the 'Isha prayer to the Maghrib prayer for the second time, and then praying the 'Isha prayer afterwards, in the case of الجمع التقديم (combining prayers by performing the second prayer at the time of the first) and shortening the prayer (Qasr)?

If someone changes their intention during an obligatory prayer or a specific supererogatory prayer to another obligatory prayer or a specific supererogatory prayer, their prayer is not valid for the first prayer because the intention for it has been diverted, nor is it valid for the prayer to which the intention was shifted because they did not initially intend it. This is by consensus among jurists.

Therefore, switching from the 'Isha prayer to the Maghrib prayer is not permissible and necessitates re-praying 'Isha due to the loss of the condition of muwalat (continuous succession) in combining prayers by advancement (jam' al-taqdim).

It would have been possible to continue with the 'Isha prayer, and whoever wished to perform the Maghrib prayer could have followed you, as the validity of following an Imam is not prevented by a difference in intention between the Imam and the follower.