The Days to Retain Paid Invoices in AR Setup Options determines how long paid invoices will appear on the invoices tab but as others mentioned above, the invoices remain in the AR Invoice History files. The maximum is 999 days thus the invoices only appearing since 2010 on the Invoices tab. (This is done automatically through Period End Processing.)
Also, when they converted to the business framework, all of the history was lumped into the last period of the prior year, whatever that was for them. For example, if they upgraded in May (and they are on a calendar year), all prior history was dumped into December of the prior year.
Why have they only noticed this now? What's their concern?