The blower motor assembly consists of the blower motor and squirrel cages; the blower motor alone is ... just the motor.
In theory you can slide the squirrel cages off the old blower motor's drive shafts and transfer them to the new motor. In reality these sometimes can be difficult to remove. Each squirrel cage is balanced by means of a small metal clip. Check to see if these clips are present on your existing cages; if not, then the clips may be lost and the cages unbalanced (OR they were within balance limits initially).
Less work is involved with installing a new blower assembly; if the price difference is tolerable, consider purchasing a complete assembly.