You'll definitely get a completely new number starting with +44 79 3700 xxxx.
During the order process sometimes silver and gold numbers will be shown. However you'll need to reload the
UM+ order page several times in order to see those numbers, as the system obviously has some problems yet.
From what I read on the "transfer credit" page (don't know the exact English translation, as I'm seeing the German version), the +423 SIM will immediately be deactivated if you transfer your credit from the +423 SIM to the new one. So I guess the +423 will stay activated if you don't transfer your credit.
At least they should let us setup a mailbox, through which we can announce our new numbers.