"I purchased Office Professional 2021 solely to continue using Publisher after retirement.

There is no supported way to force Publisher to use the Office Professional 2021 license while Microsoft 365 Apps and Office 2021 are both present on the same device with overlapping apps.

The supported guidance in this scenario is:

1. Open an Office app and check File > Account > Product Information. If it shows Office Professional 2021 or Office Professional Plus 2021, that confirms the perpetual Office 2021 license is activated.
2. If Publisher still opens as a Microsoft 365 Current Channel app, treat that as an installation/licensing conflict rather than something that can be switched per app.
3. Do not rely on trying to manually switch Publisher between Microsoft 365 and Office 2021 on the same installation. The documented guidance says these products are not designed to run side by side when they include overlapping applications such as Word, Excel, Outlook, and Publisher.
4. Use a separate environment for Publisher 2021 if continued use is required after October 2026:
   • a second PC, or
   • a virtual machine, or
   • an isolated setup / separate Windows profile where the perpetual Office version is installed
5. Keep Microsoft 365 on the primary environment for subscription features and services, and use the separate environment for Publisher.

Additional points relevant to this case:

• Office Professional 2021 includes Publisher for Windows.
• Publisher in Microsoft 365 will no longer be available after October 2026.
• A perpetual version of Publisher can still be installed and used after that date, but support for the perpetual version ends in October 2026.
• If important Publisher files must remain accessible long term, converting them to PDF or Word before that date is recommended.

References:

• Office 2021 and Office LTSC for Windows and Mac FAQ
• Microsoft Publisher will no longer be supported after October 2026
• Office Professional 2021 use in tandem with 365 - October 2026 - Microsoft Q&A
• Standalone Publisher / Office Professional Plus 2021 - Microsoft Q&A
• I want to purchase a standalone Publisher 2021 - Microsoft Q&A