The Biography for the character Weapon XI explains that Deadpool is Weapon XI by the end of the movie, and that the actor Scot Adkins takes over for Reynolds when he turns to Weapon XI. It does say right in the heading that this is not verified by IMDB, so there is some hope this isn't true, but it looks fairly convincing.
Say it aint so.... read below from IMDB
Weapon XI (Character)
from X-Men Origins: Wolverine (2009)
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In the comics there is no actual character known as Weapon XI (Weapon X for those who don't know is Wolverine). As for this character, apparently the makers of the movie took creative licenses with Deadpool/Wade Wilson's character where seems that Wade Wilson (played by Ryan Reynolds) is the merc with a mouth but he's presented with no actual scars or the Deadpool replica costume known in the comics. During the movie Wade Wilson will undergo some changes in a procedure by William Stryker turning him to a silent killing machine implanting blades in his forearms and that's when you witness his form being scared and disfigured being the character Weapon XI (played by Scott Adkins). It was revealed in a thread form Scott Adkins' fansite that he will have the last fight against Wolverine in the movie and that's what we caught a glimpse off in the official trailer where we witness Weapon XI doing a flying spinning kick in the face of Wolverine with blades appearing out from his forearms.