NISA accounts opened in 2023 will rollover into New NISA in 2024?

[adamu]

Does that mean that both accounts will be open in 2024? Can I continue contributing to both? For example once the new NISA investment limit has been achieved I could then add new funds to the legacy account? Or is the legacy account no longer open to new investments?

I think you are misunderstanding how NISA works. Check out the NISA wiki page for an explanation: https://retirewiki.jp/wiki/NISA

To answer your questions:

Does that mean that both accounts will be open in 2024?

The investments you bought in 2023 will still be in the 2023 NISA and tax free until it expires, but you won't be able to put any new money in next year. From next year, any investments will go into the New NISA.

Can I continue contributing to both?

No, only the New NISA.

For example once the new NISA investment limit has been achieved I could then add new funds to the legacy account?

No.

Or is the legacy account no longer open to new investments?

Correct.