I'm a little flummoxed in trying to move some directories around in svr2. Shouldn't the following work?

    mkdir a
    mkdir b
    mv a b

I get the following error:
    mv: b exists

I tried many of the possible variants including:

mv a b/
mv: b/ exists
mv a b/a
mv: directory rename only
cd b
mv ../a .
mv: . exists
mv ../a ./
mv: ./ exists
mv ../a ./a
mv: directory rename only

If moving directories into existing directories wasn't allowed in those days, 1) how were directories managed? and 2) when did moving directories into directories become a thing?