$ cat swap
aardvark BILL CLINTON dog
eternal CLINTON BILL forever
BILL good housekeeping BILL

aardvark CLINTON BILL dog
eternal BILL CLINTON forever
CLINTON good housekeeping CLINTON

sed -e 's/BILL/temp/g' -e 's/CLINTON/BILL/g' -e 's/temp/CLINTON/g'

$ sed 's/(BILL) (CLINTON)/2 1/g' swap
aardvark CLINTON BILL dog
eternal CLINTON BILL forever
BILL good housekeeping BILL

MORBILLIFORM OVERBILLED
TOM, BILL, AND HARRISON

awk '{ for(i=1;i<=NF;i++) { if($i=="BILL") $i="CLINTON"; else if($i=="CLINTON") $i="BILL" } }1'  swap

CLINTON BILL
BILL CLINTON
CLINTON CLINTON

awk '{
        while (match($0, /BILL|CLINTON/)) {
            printf "%s", substr($0, 1, RSTART-1);
            $0 = substr($0, RSTART);
            printf "%s", /^BILL/ ? "CLINTON" : "BILL";
            $0 = substr($0, 1+RLENGTH)
        }
        print
    }' swap

perl -pe 's/(BILL)|CLINTON/$1 ? "CLINTON" : "BILL"/eg' swap

sed -e 's/BILL|CLINTON/n&/g;s/nBILL/CLINTON/g;s/nCLINTON/BILL/g' swap