According to an AP article run on Yahoo! News:
Lawrence Roach agreed to pay alimony to the woman he divorced, not the man she became after a sex change, his lawyers argued Tuesday in an effort to end the payments.
Roach and his wife, Julia, divorced in 2004 after 18 years of marriage. The 48-year-old utility worker agreed to pay her $1,250 a month in alimony. Since then, Julia Roach, 55, had a sex change and legally changed her name to Julio Roberto Silverwolf.
“It’s illegal for a man to marry a man and it should likewise be illegal for a man to pay alimony to a man,” Roach’s attorney John McGuire said. “When she changed to man, I believe she terminated that alimony.”
If the former woman changed her name, who is the person named on the divorce decree? That person no longer exists (legally), and I would think the husband can file a motion to have the decree thrown out.