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?"

This is the recipe to have 100+ different runtimes.

Don’t forget that In large deployments, you have no idea who is using what, which features are enabled or what’s the flavor of optimization someone pushed to make things work for the customer. You have dozens of engineers and all trying to keep the lights on.

This was the state when I joined Auth0 and it took us a year to migrate every individual customer env to a managed PAAS and another year to migrate our multi tentant environments later.

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Never let your system1 to do complex migrations