A friend of mine had purchased an iPhone a few months back and shortly thereafter had dropped it. It was broken and unrepairable so she had to switch to an Android phone which was free (she was at the beginning of her contract with Verizon - full price iPhone was not an option).
The challenge in the switch was the dearth of text messages from her friends who had iPhones. Their iPhones refused to stop sending iMessages because they had a previous conversation marked as iMessage.
It was maddening. She was losing conversations and iPhone folks thought she was ignoring them.
Lo and behold there was any easy fix that was not readily apparent (after deleting contacts, conversations, recreating contacts).
All that was needed is for the iPhone folks to double tap on the iMessage (blue bubble) and a set of options appears. One of those options allows you to choose "Send as text". Do this a few times and you'll be good to go!