Quark 7 is Unicode. When using TrueType which is also Unicode, the CE characters are stored in the Quark 7 document as Unicode values. Quark 6 is not Unicode. It can only recognise character codes based on single bytes so everything except west European characters (Latin 1) will be lost.
When using PS Type1 fonts, which are not Unicode, characters are stored in the Quark 7 document as single byte codes which is the same as Quark 6.