It should usually remain without issues as the email should be connected with your overall account and not the specific service. If there is an issue Optus can reconnect / activate it on request if you have the broadband account that is required. They don't delete everything at first, just disable your access to it.
Its a good time to get 500Gb at only $3 more than the 200Gb plan at the moment.
You have to make sure they associate it with your new account. I have read of many people losing access to their email when downgrading to the NBN and I assume this is an administration issue that has not been followed on the Optus end.
@Peter49 I signed up last year as a Wireless Broadband Customer and was not offered an email account, which did not worry me as I stopped using RSP addresses.
I had a look at the CIS for Optus Wireless Broadband and there is not mention of an email account.
There are more experienced people than me posting in here, but I would look at an alternative email non Optus account well before the plan change, have all you Optus email on the new account and
if things do not go well on the plan change at least you have your email.