In the early war, the Germans readily used "inferior" Soviet armour. In a report from Zhukov to Stalin (CAMD RF 208-2511-26) on the destruction of the enemy at Skirmanovo, Zhukov writes: "By preliminary counts, in these battles we have captured 28 prisoners, 30 tanks, including 10 Soviet BT-7s, one long-range gun, 6 AT guns, motorcycles, cars, and other military items." The battles occurred on November 12-13 of 1941.
In 1942, the need for captured tanks didn't go anywhere. CAMD RF 38-11355-644 says: "Elements of the 22nd army re-captured 5 T-26es and 2 T-34s from the enemy. The tanks are functional, but lack telescopic sights. Please indicate where to receive 5 T-26 TOPs and 2 T-34 TOPs. Additionally, please advise on the procedure for receiving telescopic sights, both for domestic and captured German vehicles."
Of course, these tanks also appear in Soviet records in the late war.
From the records of the 49th Guards Heavy Tank Regiment: "25.01.44. 23 BT-5 and 3 BT-7 received from the 12th OUTP of the Leningrad front."
From the records of the 82nd Independent Tank Regiment: "At the end of 15.06.44, we received five T-26 tanks, sent from the #4 repair factory in Leningrad.
Condition of our material resources:
Undergoing repairs: 290th repair base: 3 49th repair base: 3"
However, these tanks did decrease in effectiveness, and were gradually transferred to less critical theaters. A list of equipment transferred to the Mongolian People's Revolutionary Army on February 10th, 1944, contains 13 BT-7s with 71TK radios and 12 BT-7s with no radios.
The British realized the same thing with their tanks. On April 12th, 1944, 90 Valentines in Iraq were transferred from British service to the Soviet Independent Maritime Army.
By November of 1944, it seems that these old tanks are still in service, although the M3 Lees and Stuarts are thinning out.