提供一点参考参考资料，字典中并没有详细的区分，except when中except是介词还是连词，而是给出他们的用法

Macmillan English Dictionary 2nd Edition

▪ I. except 1 UK  [ɪkˈsept] US  conjunction, preposition ★★★

        Summary

         Except can be used in the following ways:

        as a preposition (followed by a noun): We haven’t told anyone except Leslie’s dad.

        as a conjunction (followed by a clause or adverbial phrase): I’d go and see him myself, except I don’t know where he lives.

        before a conjunction such as ‘that’, ‘when’, or ‘if’: I don’t know much about the man, except that he’s a fool.

        in the prepositional phrase except for : Everything was perfect except for the weather.

    1. used for introducing the only person, thing, or fact that is not included in your main statement

        All the team were there except Eddie Stratton, who was ill.

        The shop is open every day 9.00 to 5.00, except on Sundays.

        Not a sound was heard except the wind howling.

        except for: She was dressed all in black except for a white lace collar.

        except (that): Airline officials had nothing to tell us except that the flight had been delayed.

        except when/where/what: I hardly ever get a chance to study, except when the children have gone to bed.

另一部字典：

conjunction

■used before a statement that forms an exception to one just made

只是；除了

I didn't tell him anything, except that I needed the money.

我只是告诉他我需要钱。

our berets were the same except mine had a leather band inside.

我们的贝雷帽是一样的，只是我的里面有一个皮革镶边。