It's not true, though - it definitely would have been different. Even if they were considered interchangeable for purposes of the show's premise, it wouldn't have been long before the differences in the characters of the respective empires and their histories in Trek would have shown up in writing, direction, actor and acting choices, etc. The butterflies would have done their job - it would have been a very different show. The most obvious difference that occurs to me right away is that they might have brought Tomalak (Andreas Katsulas) in as the big bad. He's a good actor, and I'm sure it would have been fine - but can you imagine DS9 without Dukat (Marc Alaimo)? The next is that I'm almost certain our "simple Romulan tailor" would have been played very differently, too...
Marc Alaimo was also the first Romulan on TNG, in "The Neutral Zone." ...In some multiverse dimension out there, he's a Romulan baddie on DS9.