New plans tend to be when you take out an entirely new contract with a different provider from the one you’ve used previously. This doesn’t count as an upgrade as you aren’t extending your contract with your current network. Usually, you’ll get a new number on a new plan but you can ask to transfer your old one.
An upgrade is when you’ve paid off the cost of your handset with your provider and you’ve taken out a new phone with the same provider, extending your contract with them. You’ll keep the same number when you upgrade.