Hmmm... I'm not sure this is DLA that you're thinking of, but Incapacity Benefit, as that was the previous benefit that the DWP commonly paid Income Support instead of in the past - it was on it's way out 7 years ago, but still there. There would never be a scenario where the DWP would pay Income Support instead of DLA.
DLA is also not a Taxable Benefit, where Incapacity Benefit was.
From your description above, I'm not 100% sure what has gone on with your current claim - Are the DWP saying you are not entitled on medical grounds, or is it that you have somehow been paid contributions based ESA which has now ended?
Either way, the decision taken 7 years ago would not affect the current one I don't think, as the rules (both regarding criteria for limited capability for work, and indeed contribution based benefits) have changed. You have taken the right action in requesting the Mandatory Reconsideration, and I would just treat it as a totally separate matter.
If you really think there was an issue with a DLA decision, then I think you need a specialist to look at the paperwork you have to try and figure out what happened. Maybe try a local CAB or Welfare Rights.
Hope this helps.