At present we have a system for creating Mechanized Corps, the Soviet Army's strongest unit, where you must first build mechanized brigades and form them into Corps. Due to the poor quality of troops formed in this way the resulting Corps is not really useable for some time. More importantly, this is does not reflect the historical reality of their evolution from Tank Corps.
Mechanised Corps formation:
1st - from 27th Tank Corps, September 42
2nd - formed September 42; became 7th Guards Mechanized Corps, July 43
3rd - from 8th Tank Corps, September 42; became 8th Guards Mechanized Corps, October 43
4th - from 28th Tank Corps, September 42; became 3rd Guards Mechanized Corps, December 42
5th - from 22nd Tank Corps, September 42; became 9th Guards Mechanized Corps, September 44
6th - from 14th Tank Corps, November 42; became 5th Guards Mechanized Corps, January 43
7th - formed January 1943 (was slow to form not being fully formed until September 1943)
8th - formed February 1943 (also slow to form, not fully formed until October 1943)
9th - formed January 43, but didn't become fuly formed until September 1943
10th - formed December 1944 in the Far Eastern Theater. Wasn't fully formed until June 1945
(13th) – actually the reorganized 13th Tank Corps November 42, became 4th Guards Mechanized Corps, January 43
Though the 2nd and 7th-10th MCs appear to have been formed as fresh formations, it should be valid to convert an existing TC to an MC. A reduction in experience levels would be reasonable to reflect the costs of reorganising.
The creation of Guards Mechanised Corps provides another mechanism for creation of Mech Corps:
Guards Mechanized Corps
1st Guards - from 1st Guards Rifle Division, November 42
2nd Guards - from 22nd Guards Rifle Division, October 42
3rd Guards - from 4th Mechanized Corps, December 42
4th Guards - from 13th Mechanized Corps, January 43
5th Guards - from 6th Mechanized Corps, January 43
6th Guards - from 3rd Guards Motorized Rifle Division and 49th MechanizedBrigade, June 43
7th Guards - from 2nd Mechanized Corps, July 43
8th Guards - from 3rd Mechanized Corps, October 43
9th Guards - from 5th Mechanized Corps, September 44
Promotion from an existing MC was common but exceptionally an existing Guards rifle division could provide the founding unit for mechanisation.
Game Balance: there may be a concern that easier routes to Mechanized Corps formation would lead to masses of Soviet MCs dominating the battlefield in '43. However this is already prevented by the primary restraint on mechanized warfare for the Soviets : their lack of trucks. In fact it is difficult to replicate the historical Soviet OOB of ~15 Mech and 29 Tank Corps without crippling the supply situation.
(Source - a Dupuy institute discussion that pulls together information from sources such as Erickson, Sharp, Glantz and Zetterling)