Start creating new feature in system2 and over years let it slowly take over system1.
Whenever new feature is published in system2 don't forget to tell your customer "You see how fast we delivered this new feature, in old system1 it would take at least 2 upgrade cycles. Oh btw, this other new feature your just mentioned do you want it in system2 next month or in system1 in Q3/2025?"
Whenever new feature is published in system2 don't forget to tell your customer "You see how fast we delivered this new feature, in old system1 it would take at least 2 upgrade cycles. Oh btw, this other new feature your just mentioned do you want it in system2 next month or in system1 in Q3/2025?"