All people have to do is before they change to an Android, turn iMessage off on your phone.
It will remove your number from the iMessage check, and It will stop other iPhones detecting your number as an iPhone.
People switch over phone's, leave iMessage on, and now anyone who tries to text you on an iPhone will text through iMessage instead of a Text Message. I'm surprised Apple doesn't know this
I did it when I switched from my 3GS to Note 2.