A company I bought a plugin from, had a one fixed price for all time. They kept all the old customers in that bracked, but new customers have to buy a yearly fee.

So I would create a new subscriber role and simply let your new customers pay a bit more.

It is wise to be upfront with existing customers : say you have need for more funds or more investments, relay that to your existing customers, say you are keeping their price (indefinitely if possible) and that new customers going to go into the new tier.