ZFW controller described confused flight plan status event when equipment problems precluded a normal automated hand off and incorrect assumptions were made regarding an aircraft's IFR vs. VFR status.

Narrative

Oklahoma City Approach called me on the shout line after I accepted the hand off on Aircraft X via automation. They told me that they were showing an Interface Failure for Aircraft X; and I told them that I had taken radar on the aircraft. They told me that he would be climbing to 080. The data block of Aircraft X indicated that he was VFR; and approach did not indicate that the aircraft would be IFR at 080. Upon initial contact with Aircraft X; I asked him to confirm his squawk code. The code he was squawking; which he also verified when I asked; was in use by another aircraft in the ZFW system. I concluded that this was the reason that OKC Approach was showing an interface failure; and that I needed to put him on the code that my URET showed him being assigned. I issued the new code to the aircraft. I gave a relief briefing. During this briefing; while going over traffic in the sector; I noted to the relieving Controller that Aircraft X had been at or around 080 more than he had been at 085 which would have been appropriate for VFR flight. I told the relieving Controller that he may want to ask him if he knew that he was at the wrong altitude for VFR aircraft. Recommendation; later in my shift; the Manager called me to his desk and informed me that Aircraft X had in fact been IFR for his entire flight; and explained that I should have been more inquisitive as to his curious altitude. My thought is that VFR aircraft often maneuver around their appropriate VFR altitude to stay clear of clouds. In hindsight; I could have been more proactive and queried the aircraft about his altitude.