Old SG2 is G3Si1 (EN)
Old SG3 is G4Si1 (EN)
The technical difference is the chemical analysis.
The G4Si1 is a little bit stronger, so it is prefered when welding thick plates or high strength steels.
You can see directly checking the wire analysis, for example the Manganese (Mn),
G3Si1 must have from 1.3-1.6%,
G4Si1 must have from 1.6-1.9%,
The same is with Silicium.
APPROVED BY TUV, dB.