Good to know. I guess Bulgaria-Con would have the same problem

Ehh, what, it should succeed in 2 moves.

Ahh, now I get it, you mean if 2 units can swap places at the same time if one is going from NC to a region and the other is going from the region to SC....

Yes, and apparently it is viewed as swapping places which isn't allowed

Yup, definitely not allowed. Specifically mentioned in the rulebook, all versions, if I recall correctly, even.

And yes: even F Bul/sc - Con, A Con-Bul would be failed moves.

It's not in all versions of the rulebook - earlier ones didn't have this rule - but it is in most. Richard Sharp's "The Game of Diplomacy" covers this briefly:

"Another clarification provided by the new (1971) rulebook concerns units changing places. In the ordinary way, two units cannot exchange places (‘Changing the Guard’), even in cases where logically they would easily be able to do so: e.g. F(Spa)(nc) - Por, F(Por) - Spa(sc), the so-called ‘coastal crawl’, is not permitted to succeed, though in reality the fleets would not come within a hundred miles of each other. This is a rare case where the revised rulebook went against majority opinion - most GMs before 1971 allowed this move. However, if one (or both) units are convoyed, both moves succeed: A(Ank) - Con, F(BLA) C A(Con) - Ank. This latter ruling is often useful in the Denmark area (see ‘The Loop’)."

That sounds like its straight out of the Ankara Crescent rulebook. Not the original 1964 Stockholm Agreement, but from the second appendix to the 1968 Havana Corollary.

Actually, now that I think of it, that was in reference to a fleet swap in Moscow (SC), which is, of course, explicitly permitted and even encouraged in the rulebook:

"RULE 6.B.14: SWAPPING UNITS

Under ordinary circumstances, two units are not allowed to exchange places ('Changing the Guard') EXCEPT in cases where two units is merely a stop in reaching a unit exchange consisting of three or more units ESPECIALLY in cases where they would easily be able to do so. For example, Spa (SC) is hundreds of miles long in real life, and limited occupancy to a single fleet is capricious and silly. Therefore, occupying a coast with a single fleet is prohibited UNLESS said single fleet is supporting another fleet to the same province EXCEPT if said support is to, and, or of the south coast of Moscow, in which case the Caspian Sea needs to be occupied IF AND ONLY IF it is not a federally recognized bank holiday. What? I ordered a coffee an hour ago, why can't I stay in my seat? What do you mean 'wasting space'? Look, I paid two American dollars to sit in this restaurant and I'd rather not have my rights viola..well if you're doing to call the police, I'll take fifteen cents worth of coffee, thank you very much. Now, what were we talking about again? Oh yes, fleet swaps. These are totally legal."

@captainmeme Oh, interesting :-) Thanks for the clarification.

In other words, captainmeme, I'm showing my age. Thanks for that info though.

Yeah, we're still debating whether CaptMeme is not by now too old to be allowed on the site.