Hi,
Your existing UTR will not change. You will keep it for life, in most circumstances.
Your rental income will still be declared on SA105 and self employment on SA103. To register as self employed, please contact the National Insurance.
National Insurance: general enquiries
They will only register you if you have a valid National Insurance number (NINO). If you do not have a valid NINO, you will need to apply for one:
Apply for a National Insurance number
Your current gateway account should not change.
Thank you.