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A 5g sim will work on 4g fine.
All new o2 sims are 5g compatible so you can upgrade all your sims if you wish at the same time.
The sim that arrives will be blank so you can use it to swap your partners number to Guide: Sim Swap: a mini guide 2017 update
You can just keep using your current sim in your current phone.
she just needs to start the sim swap from her old sim card.