My husband was on an iPhone for two years, last iOS was 7.1.1
On May 1st, he changed to a Samsung Phone S4. On May 7th we realized he was "not" receiving any texts from people sending from their iPhone, but "is" receiving all texts from non-iPhone receipents. The iPhone has not been formatted yet, so I went into Settings and turned off all aspects of iMessage since the texts were still coming through to it as of May 6th.
I got on the phone with Verizon tech support who told me that once the iMessage area was turned off, it would take about 24 hours for iMessage to realize texts were not going through and that after that the iMessages would be sent as SMS messages.
It has now been almost 3 full days since the iMessage on the iPhone was totally turned off and we are still "not" receiving texts from other iPhone users. My husband uses his phone for his business, so all of this is critical.