At my age I can tell you first hand that people very often change their opinions about enormously important issues as they get older (and hopefully wiser), so I don't think it is unreasonable for Shannon to have changed her mind about immortality. I'm sure you can find a way to continue this as a dynasty and explain her change of heart (e.g., outside influence as Audren mentioned; as a way to redeem her family after their last attempt; or her earlier opinions were based on jealousy if she was not destined to be an immortal, for example.)
To me, continuity with previous attempts isn't all that important - I view each dynasty as its own entity. If you're on track with this one and like the sims you are playing then it would be a shame, in my opinion, to convert it to a legacy. Either way I'll continue to read it, as will many others, I'm sure.
