It was my understanding that Microsoft pulled the rug out from developers multiple times by making new OS versions not quite backwards compatible.
rewrite your windows mobile 5/6 apps for windows phone 7? ok, its a brand new platform, i get it.
rewrite wp 7 for windows phone 8? ok... (there may have been wp7 mango that needed tweaks but my memory is too fuzzy)
rewrite windows phone 8 for 8.1 ? cmon now
rewrite windows phone 8.1 for wm 10? no