I would like to know some information before launching my Tor node. If my node is assign as a guard node, can it still be used as a middle node or has it became a permanent guard node?
All relays are suitable for all positions in ciruits. How clients use them depends on their assigned flags, with the exception of exiting traffic which is defined by their chosen exit policies.
So you will potentially act to some users as a guard and others as an intermediary (not always just the "middle") and possibly an exit to others (if you have a policy that allows it).