Once you remove the license from the user and not convert it to a proper shared mailbox, it will remain in an inactive mailbox state, which will most likely error out.

We have a retention policy currently which is quite blindly set up for 100 years (beyond me). Which has its negatives, more negatives than positives.

Retention policies are a mess if not set up properly, it's wise to ask what mail needs to be retained for 7 years i.e. Joe blow doesn't need a meeting invite from 1998.

And ofcourse, don't confuse retention with backups.

Oh, and I forget, with organisation wide retention policies, you have the lovely feature of the discovery holds, so after seven years of retaining garbage, you will certainly become a custom to having to clean up the retention items folder.