tanu,
I'm taking the very simplistic view that an approval before PD 2007 is an "upgrade".
I know some 2007 PDs are definitely "upgrades" but some of the pre 2007 PD are probably longstanding "straight" cases.
I don't intend to analyze it further than that.
Currently, about 46% of approved cases in the Tracker have a PD earlier than 2007. On that basis about 1.0 - 1.2k are "upgrades".