Share tariff

[Jmc123]

Increasing to 1gb data ,can I reduce my other phones tariff and use this instead?

[Cleoriff]

Info re Sharer tariffs here. http://www.o2.co.uk/sharer-plans (Not sure you could reduce your tariff on the other phone though?)

[MI5]

Basically no.
You need a lead tariff then you can add other contracts at renewal time but not part way through.