An upgrade is, by very nature, a change of phone while keeping your existing number. Or do you just mean a change of payment-plan? This is where the "upgrade" terminology can be a little misleading.
With a physical SIM, you simply move your working SIM from one phone to another - a simple 2 minute job. With an e-SIM, you now effectively have to deactivate a working SIM and activate a new replacement one. That "can" work whilst abroad, but does come with a risk of loss of service. Only proceed if you are prepared to accept that risk.
If you're just changing payment plan, there is no reason to replace a perfectly working SIM - this is an account-level change.
