sed 's/_/-/g'

awk 'BEGIN{
    q="\047"
    path="/path/to/search"
    cmd="find "path" -type f -name \"*_*.jpg\""
    while(( cmd|getline f )>0){
        m=split(f,file,"/")
        gsub(/_/,"-",file[m])
        newfilename = join(file,1,m,"/")            
        mv = "mv "q f q" "q newfilename q
        print mv
        #system(mv) #uncomment to use                     
        close(mv)
    }
    close(cmd)
}
function join(array, start, end, sep,    result, i)
{
         if (sep == "") sep = " "
         else if (sep == SUBSEP) sep = ""
         result = array[start]
         for (i = start + 1; i <= end; i++)
             result = result sep array[i]
         return result
}'