does upgrade from Mojave to BigSur disturb old word or office

Go to the Apple menu and select About This Mac ...

Then select System Report

Select Software, then select Applications

Scroll down until you see Microsoft Word. Click on Microsoft Word once, you will see some information about Word and there it will say

Kind: 64-bit

if it is 64-bit. If it says 32-bit, it will not run under Catalina or under Big Sur.

The only supported 64-bit Microsoft Office solutions on macOS are Microsoft 365 subscription and the single-purchase Office 2019 for Mac — on macOS Mojave (10.14.6), Catalina (10.15.7), and Big Sur (11.5.2). Your older 32-bit Office 2011 suite applications will not even run on the last two operating systems. Your documents are unaffected by the 64-bit requirements.

The net of the preceding paragraph is that you need to either subscribe to Microsoft 365, or do so until the next Office 2022 for Mac is released this Fall, and then drop the subscription and purchase the new Office suite. Your choice, especially if you need Outlook.

Alternatives may be the paid SoftMaker Office or the free LibreOffice 7.2 Suite. Both will open and save your existing Microsoft documents, and neither have an email client.