I'm not following.
Say the app is originally $10.
A year later you push out the "2013 feature pack" for $10 more.
Existing customer pays $10 ($20 total over lifetime) for upgrade. New user pays $20 at once ($10 base + 2013 pack).
A year passes, 2014 pack comes out, existing user pays $10 ($30 over lifetime), new user pays $20 ($10 base + 2014 pack (which subsumed all features of the 2013 pack)).
The existing user is always getting an at-the-moment discount.
Now, yes, the existing user pays more over the lifetime than the new user, but this is no different than conventional upgrades. If you've religiously bought every Lightroom upgrade you've paid more than the guy who jumped in at Lightroom 4 over the lifetime of the product, even with the upgrade discounts.