Deleting a user will cause any records that are assigned to them alone to become unassigned (and a reminder message about this will appear when you try to delete the user to double-check that you want to go ahead).  

If you're happy with this, fine. Alternatively, you might like to consider using one of the following approaches:

• Instead of deleting the record for the user who's left, simply disable their login by removing the tick from the checkbox next to Login enabled on their user record. This will leave their name in the Assigned To field of any records already assigned to them but will prevent them from logging into Workbooks. 

OR

• If someone has replaced the previous user, rather than creating a new user record, you can open the previous user's record and overtype their name with that of the new user, as well as replacing their login email details with those of the new user. This will result in all the records that were previously assigned to person A now being assigned to person B (including records that were created before person B started to work for you).

NOTE:  BE CAREFUL BEFORE DELETING A USER. Do you have any processes that run as this user? Have any web keys been created based on this user? If a user cannot login, any processes that runs as them will fail. Similarly, if a web key is based on a user whose user record is deleted or login is disabled, the auto-generation of Sales/Lead records will fail.