Green’s Dictionary of Slang

(US prison) to take alternate active/passive roles in anal intercourse.

1971–2		C. Shafer ‘Catheads [...] and Cho-Cho Sticks’ in Abernethy Bounty of Texas (1990) 216: swap out, v. to alternate the male and female roles in unnatural sex acts.
1979		E. Torres After Hours 59: Lotta fags out. [...] In the Barrio, ‘malas lenguas’ said that Walberto and Monchín was swappin’ out. Of Walberto, I believe anythin’, he’s a natural-born pimp.