I'm not sure I get your question. To keep using your existing phone (Z10) on your existing line, you will continue to pay what you are currently paying. You said you recently "upgraded" - to what phone? Do you now want to g back to the Z10 instead of the newly upgraded phone? You can do that - and just keep paying what you pay now.
If you are on a DPP for the NEW phone, you will continue to make those payments also, regardless of what phone is active on your line.