This has probably gotten covered- but I couldn't find it in a search.
Do K2 scripts work in K3?
Yes, they do. You should be aware, however, that there are slight differences in filter behavior between K2 and K3, so programming that is extremely sensitive in this regard (such as Garritan Stradivari Solo Violin) may not work as expected.
Be aware also that instruments with heavy scripting that were created in K2 often do not function correctly (or work at all) in the K3.5 beta. You should be able to work around this by opening the instrument in v3.0 (actually any version prior to 3.5) and saving. After doing so, they seem to work in 3.5