I am not sure what you are asking. TB can handle multiple accounts, such as one for Centurylink and one for Frontier, but I don't think that's your question. To keep her centurylink address, she needs to keep Centurylink as an ISP. Does this help? If not, please share more information.

Thanks David but the problem is that CenturyLink does not serve Connecticut, so we apparently cannot sign her up for Internet service here. I spoke with a rep a few days ago, and she said that my partner owns her email address forever, and all we have to do is assign it to another ISP. I understand that a CenturyLink address cannot be assigned to another carrier, but have been told that Thunderbird can handle multiple ISPs, which would permit her to keep her old email address while using Frontier’s Internet services. We just want a way to allow her to not have to notify all her email contacts and 10 years of saved files.

Fundamentally this is a question for Centurylink. Will they continue to offer that email address for whatever payment per month. In the end Thunderbird is a client, so it will need to access the Certurylink system and find a valid email address that is accepting email. That will not happen once the account is closed.

Some years ago My ISP in Australia did continue to offer my email access for $2 a month per address, but in this day, with ISP's distancing themselves for things like email I doubt that will be the case, but there is no harm in asking.

Fundamentally I would suggest changing to another provider. Be it free like Google or GMX or paid like Mailfence, Protonmail or Ghandi. Being locked to an ISP provided email address has issues as you are starting to find. It also makes it difficult to switch to a competitor when they offer a good price. Personally I recommend everyone go out and start weaning themselves from the ISP provided email address onto something not linked to the internet provider. Now you may only have a choice of one provider, but that will change, if from nothing else than starlink.

The mail from the old account can be archived in Thunderbird (just not in the original account as it will go away with the account). But storage in the "local Folders" account is permanent, if locked to the devices hard disk.

It is not all that difficult to email everyone in the address book with a new mailing address, but the longer it is before you close the account you do so, the more chance you have of picking up those that did not listen. I suggest for mass mailings like a change of email address you use the mailmerge addon as it will allow you to send individual emails to each address book entry advising of the change. No bulk CC or BCC that might run foul of a spam filter.

You should also start soon after you notify everyone using a filter replying to all new mail in the account with a reply (use a template) stating the account will be closing soon and what the new address is.