Why was the Lord’s Supper changed from the first-century practice of being a part of a meal?

Answer:

We start out with a problem. In order for something to have been changed, you must first prove that it was done differently. Claiming that the Lord's Supper was originally a part of a common meal isn't proof. Instead, I can prove from I Corinthians 11 that the Lord's Supper was separate from common meals. See What do you think about partaking of the Lord's Supper after dinner? In this answer, I addressed the passages typically used to justify making the Lord's Supper a part of a common meal and show that the passages are being misused.

Why you claim this imaginary change was connected to pagan practices, I don't know. In my reading of history, I find that many pagans used meals to celebrate their gods.