Possibly. I'm no expert, but it seems like if you were using an incorrect password, you would be running through every old system every time, regardless of if

Possibly. I'm no expert, but it seems like if you were using an incorrect password, you would be running through every old system every time, regardless of if the person had upgraded. The only time you would short-circuit any of the methods would be if you used the right password. I actually think the originally described system is more vulnerable to a timing attack, since you could determine which crypto system a user's password is using based on the timing.