"Honestly, you have no idea what's entailed. I'll leave it at that."
Honestly (and obviously), more importantly, neither do the people planning and executing the upgrade, (nor did they with the prior upgrade attempts or the restore for the that matter), hence the delays. THEY are the ones who should know what was entailed, not your customers! Had they done the analysis to estimate the effort properly, we wouldn't be talking about the 3rd? (I've lost track) delay for the completion of the current project, now into 2013, nor the abandonment of the previous upgrade attempt. Right?
Do you have a certified project manager running this?