Switch SIM to new iPhone 12 Pro

[SouthofNorth]

I am currently on a refresh contract with an iPhone XS Max. 

If I were to buy an iPhone 12 Pro outright from Apple, can I just take the SIM out of the XS Max and put it into the iPhone 12?   Would it work straight away? 

Do I need to speak to O2 about my contract? 

Thanks

[MI5]

@SouthofNorth 

Yes it will work but the max data speed will be 4g.

If you are in a 5g area, you can calll into your local o2 store for a free 5g sim swap.

There's no need to tell O2 that you are changing phone.

[SouthofNorth]

Also, is it cheaper to buy a device outright from Apple and use the O2 SIM or buy the device on contract from O2? 

[MI5]

@SouthofNorth 

Your airtime will be cheaper / better value if you just have a sim only tariff and buy the phone outright.

[KernelPanic]

I can concur with that.  I am O2 sim only (100GB and unlimited texts/mins for £20 per month) and have purchased the iPhone 12 Pro direct from Apple.  It works out approximately £9 cheaper per month and you get more data allowance.