probably, i used to have an eclipse MK2 dropline which my antisyphon fitted perfectly, since then i have started using an in-line reg and my bottle's anti syhpon is is now upside down, so yes you it needs to be changed, OR, try inserting an o-ring into the bottomline before you put the bottle in, this will stop the bottle goin in as far, by say, half a twist thus producing the anti-syphon back the right way up.
catch my drift?
HP