Creating the new monitor does not harm the old one.
But now your system no longer uses it, it will be deleted automatically from your profile at some stage. I forget how many weeks of inactivity causes that.
There are two reasons for deleting the old one yourself.
One is to release thinkbroadband system resources which otherwise continue to poll that address all the time and update the database. As an old-school professional IT man that is a deeply ingrained thing to do
The other is that someone else will soon have that old address. Maybe already. The thinkbroadband system will be pinging it continuously, and the new holder's system (firewall) could take it as attack probes. Not likely to cause a problem, but if that person examines the logs they may wonder what is going on.
If you were in that person's place, would it worry you?
Kindness isn't going to cure the world of all its awfulness but it's a good place to begin. Daisy Ridley.
My broadband basic info/help site - www.robertos.me.uk.
Domains, site and mail hosting - Tsohost
Connection - AAISP Home::1 80/20. Sync 65618/13914Kbps @ 600m. BQMs - IPv4