It was changed between 3.1 and 3.2:
This is a terse description of the new features added to bash-3.2 since the release of bash-3.1.
Quoting the string argument to the [[ command's =~ operator now forces string matching, as with the other pattern-matching operators.
So use it without the quotes thus:
i="test"
if [[ $i =~ 200[78] ]] ; then
echo "OK"
else
echo "not OK"
fi