MiG-31 upgrades at Sokol 'continuing' in 2004?

Sokol Aircraft Plant to Continue MiG-31 Upgrade in 2004

“Despite complications with financing, the Sokol plant is able to function,” the general director of the enterprise, Mikhail Shibaev, declared on Friday at a ceremony devoted to the plant’s 72nd anniversary.

The Plant has produced more than 43,000 airplanes during its history. According to Shibaev, the Yak-130 training fighter was created in 2 1/2 years, and 2 airplanes already have been built from this batch. Delivery to India of the MiG-21bis airplane is continuing. Mikhail Shibaev emphasized that in 2004, the plant is supposed to carry out all obligations. Moreover, Sokol will continue work on the upgrade of the MiG-31 for the Russian Federation air force and the MiG-29 for export deliveries. The Sokol plant also has completed assembly of a batch of airframes (body without the filling) for the SF-260 airplane on order of one of the European firms.

Source: 31.01.04, RosBalt

What MiG-31 upgrade is it referring to? Work on the MiG-31BM, or modernization of existing MiG-31s to MiG-31BS standard?

(I also assume that the MiG-21bis deliveries are of MiG-21UPG Bison aircraft).