upgrading

Yes once you've upgraded and paid your other contract up the phone is yours , and yes you can keep your old number and use it in your new phone.

As long as your new phone is either network unlocked or from ID it doesn't matter about putting your old sim card in your new phone.

I swap my sim cards around from phones without issue 

@Lovy  your upgrade new phone will arrive via the post without a new SIM number!

You just need to insert the old SIM into the new phone turn it on, connect to wifi wait for any network or software updates for that mobile, input your data then your good to go…

You do no need to register the SIM or activate anything!
The SIM and contract continues as before

Just to clarify; Contracts and numbers are allocated to SIM cards not the actual mobile, these are transferable to any mobile that works on the ID Mobile Network.

@Lovy if you need any more help when the mobile arrives, like transferring data over or anything, just pop back in to the forum!

While I was filling in my details for upgrading my phone to new one after end of contract the form filling got cut off just after I entered my payment details. Please advise

Thanks for your trouble….