Exchange Online - How do I disable email functionality, but still retain access?

The HR guy at our office recently passed away. He dealt with quite a few online portals for things like state taxes, insurance etc.

We would like to simply retain access to his mailbox but loggin in via outlook online.

How can I:

1. Remove his license, and move it to a new hire.
2. Retain access to login to his mailbox via outlook online
3. Prevent others from emailing his email address.

I think what needs to be done, is somehow convert his mailbox into a shared mailbox, which takes no license, and give delegation to a manger so they can log into the mailbox itself to get old emails etc. Is that possible?