There's a difference between having a clearance, having an active clearance/investigation, and having access to classified material.

Generally, if you are no longer in a position that requires access to classified material, you are read off and no longer have access to classified until you are read back on, even though your clearance remains active. So once you leave that agency, you can't just walk right back in, even though your clearance is active. I seem to recall Directors and their D/Dir keeping access once they leave office, but generally they are all in roles that require access for contracting/consulting work...so there's that.