Remove phone number from iMessages
So I recently switched from an iPhone to an Android. I have factory reset the phone and linked it to someone else's Apple ID, removed my phone number from my account and de-registered the device (I have called Apple Support twice to confirm this) but still cannot receive text messages from iPhone users. I have done some testing and found that if someone who has never texted me before on an iPhone texts me, the messages are green (SMS) and I recieve them on my Android but for friends who I constantly text and already have a conversation with the messages are blue (iMessage) and the messages don't reach me. Before I de-registered my number and device the texts got redirected to my iPad or iMac but since I de-registered it they go nowhere, not my Apple devices or my Android. I have called Apple support and they told me to tell all my friends to delete their previous message convos with me and start a new one. There has to be an easier way, I don't want to ask them to delete entire conversations because some of them are group texts and it doesn't seem fair to ask them to do this when it's really my problem. Also, it's quite inconvenient to text EVERY single person I know who has an iPhone and has talked to me before. There must be something I can do on my end. Thanks in advance!
iPhone 4S, iOS 7.1