Copy paste of details on current one between systems and done.

Same SIM but look at the OneNZ Prepay plans on the website

If you pick up a new SIM then you might be eligible for the prepay joining offer before then porting in your Pay Monthly number?

https://one.nz/legal/terms-conditions/double-top-up/ 

https://one.nz/legal/terms-conditions/5gb-sim-bonus/ 

They are doing payment flex not actually porting 

Just make sure that they don't enforce the 30 days notice when going from postpaid/on account to prepaid :) 

UPDATE:  Finally got round to this matter (had been overseas etc etc).

Went into retail store, and they did all the 'paperwork' there and then.  Yes, had to stick with the 30-days notice, but got a healthy rebate via part charges due to the cross-over date being halfway through the monthly plan.

On the appointed day, monthly plan calling went dead, so topped-up as expected. Had to reboot phone for Prepay option to be recognised. 

ONLY hitch was a subsequent TXT saying I was now on Prepay 'casual rates' instead of the expected set amount plan.

Waited 24 hours with no change so went into a different OneNZ store (was away from home) and asked for assistance .... guy instore took phone number, asked for confirmation of name etc, and pronounced it "done" ... three minutes at best 😁

You could of added the base plan in 5 seconds using the OneNZ App

@Linux ... yes, always easy when 'you know how'.

Was also touch concerned that the top-up may have gone astray.

No great problem overall though.

Don't forget to have enough balance in 30 days when the base plan renews or you will lose any carryover minutes & data

     >Don't forget to have enough balance in 30 days when the base plan renews or you will lose any carryover minutes & data<

Yes, thanks for that ... reminders set 😊

Seems one can top-up any amount, which subsequently acts as a pool from which regular top-ups will automatically be drawn .... up to 12 months apparently.