@JanS2000, When you delete a user from within Hornbill, any Request, Asset, Service Subscriptions etc. that has been associated, you should assume that the link will be lost. In some cases, if you re-import a user with the same User ID, then some associations to entities such as Requests, Assets Ownership and Service Subscriptions will be re-established, although this is not guaranteed. If you have changed the User ID, all links must be established manually.
To retain the links, you would NOT delete the record within Hornbill and modify the Import to use a different field to identify users uniquely, for example, the Login ID field. Of course, if the login ID is also changed, you must manually modify this field in Hornbill to match the new login ID being imported.