[@netweb Keymaster 10 years, 9 months ago]

When importing every imported user has there old user id added to the wp_usermeta table:

• _bbp_old_user_id 318

You can check them all out via this MySQL snippet:

• SELECT * FROM wp_usermeta WHERE meta_key LIKE '%_bbp_old_user_id%'

So, yes, preserving/storing the old user ID is already included in bbPress