Neither 16-bit HSO samples sold together with C5 nor those of a full boxed version that were both 16 and 24-bit are suitable for running in HSSE. The former product lets you use its license with the newer HSO sound set developed for HSSE . The latter, as far as I know, doesn’t. In any case, you can’t just use the old HSO samples in HSSE. You have to get that newer sound set developed for HSSE at the time of C6 release first.
When you say “the previous HSO I got when purchasing Cubase 5” do you mean you bought the license for a 16-bit version of HSO that came with C5 or an HSO version that was was sold separately in a box and included both 24 and 16-bit samples?