Russian designations are complicated, as it's not always the
manufacturer, the air force and the ASCC agree to what is a new
type or variant.

The ASCC used to designate trainer versions of fighters
differently, for example:

Su-7            Fitter
Su-7U           Moujik

but starting with MiG-23 they have had the same code name as the
fighter version.

Examples of where the manufacturer and air force have different
opinion of what's a new type and a variant:

Manufacturer    Air force

An-2            An-2
An-4            An-2ZA
An-6            An-2V
MiG Ye-5        MiG-21
MiG Ye-6,7,8,9  MiG-21 with different suffixes

But it gets worse as:

MiG Ye-7        MiG-21PF Type 76
MiG Ye-7        MiG-21FL Type 77

And sometimes it's the makers designation that's well known, as
in the case of Tu-95:

Tu-88           Tu-16
Tu-95           Tu-20
Tu-22           Tu-22
Tu-26 (?)       Tu-22M   <- this was for budgetary reasons

Sometimes, the designations change for almost incomprehensible
reasons. 
The MiG-29M was at one time almost called MiG-33, but it couldn't
officially get that designation until adopted by the air force,
and now it seems like it won't ever.

The Su-27IB first got renamed Su-34, then Su-32FN -- which was
completely incomprehensible, until we learned that the Su-25TM
had been re-designated Su-34.
The military trainer developed from the Su-26/29 series was also
designated Su-32 for a while, but now it's called Su-39.

And now we hear that the Su-25TM when developed will be designated Su-39.

And so it goes...