If you've got a high growth rate and you're improving process quickly enough, this is essentially an ethical Ponzi scheme.

Previous_gen_users << current_gen_users

Ergo, take a one-time charge to upgrade your previous gen users to current, with the revenue from your current gen growth.

Key points here are probably: (1) that it's unexpected & doesn't create an expectation (unsustainable), & (2) that it's unannounced & properly timed (manage the Osborne effect & avoid screwing customers who choose to pay to upgrade themselves ASAP)

It won't fit all business models / products but seems practical in most cases.