Costner is too old
This movie could have been successful if they got a younger actor.
Make it so that Ryan Reynolds' memories can only be transferred to a close family member. His brother is in a special care facility. The brother could be played by a Ryan Gosling type. Transfer the memories to the brother...who instead of being hyper violent and psychotic is "slow" or has some other mental issue...but with the transfer he is normal.
Or I guess you could keep him as a criminal...but the important thing is to replace Costner with a younger actor with a less effing annoying performance. I'm pretty sure I liked Costner's character the least...I didn't want him to have a happy ending.