The dis/incentives are based on welfare and local goverment pensions within that state.

They're not proposing 'enforcing' it at all, 'just' disincentivising it; which will equally affect people moving into that state regardless of whether they were before they had a third child or not.

A lot of people (hundreds of millions) in rural India will never leave their state of birth to settle elsewhere. Ties to land are too strong and cost of moving is much higher than western countries. The attitude towards free movement is just not like what we are familiar with.

Presumably if you ever came back to the original state where 2+ are banned, they'd eventually figure out that you have a third child, no?