to a new subscriber, that new subscriber could potentially log into your old Telegram account (which would now be a new, empty account with no history, as your data moved to your new number). However, if you had logged in on other devices with the old number before changing it, and those sessions were still active, there could be a very small risk, though Telegram usually forces re-login. The main risk without 2FA is someone else claiming the number and creating a fresh account.
Notification of Reset: If someone tries to reset an account associated with a number that now belongs to them, Telegram will send a notification to your current active Telegram session(s) (linked to your new number) indicating that an account reset process has been initiated. This gives you a window of time (usually 7 days) to cancel the reset by changing your number again or proving ownership.
What Your Contacts See:
Automatic Update (for Mutual Contacts): If your contacts have your old number saved in their phone's address book and have "Sync Contacts" enabled, Telegram will often automatically update their display of your contact to show your new number, provided your privacy settings allow them to see your number.
Notification to Mutual Contacts: Telegram will telegram number database typically send a small service message notification to your mutual contacts (people who have your old number saved and who you also have saved) letting them know you've changed your number.
Privacy Settings Apply: If you had your phone number privacy set to "Nobody" or restricted to certain contacts, those settings will continue to apply to your new number.
In summary, when you properly use Telegram's "Change Number" feature:
Your Telegram account and all its data remain safe and intact.
The primary security risk to your account with your old number comes from not.