I know this show isn't generally too concerned with realism, but I think Dylan being a different girl would be less believable than the already semi-ridiculous notion that her memory has been wiped.
I would think if she was as close to Julie as they claim, Julie and Susan would have photos of the two of them together.
Even if 12 years or so has passed I would think they'd be able to distinguish whether or not it was the same girl.
This show really can be ridiuclous at times though.
I'm sure they'll come up with whatever explanation they fancy and if it doesn't make sense there'll be enough other stuff going on that people will soon forget this storyline.