Does anyone know how to assign variables within variables?

## assign breast size
${breasts=huge breasts} # assigning variable

## adjust breast size
${adjustboobsize=${breasts=small breasts} # reassigning variable
}

##
## call variable to reassign breast variable
${adjustboobsize}

## prompt
1girl, solo, ${breasts}
The thing just blows up if I call ${adjustboobsize} and I think I get a syntax error. Anyone know what I'm doing wrong or if it's even possible?