If you're going to ask me, the mere fact that Globe could not provide you with contracted speed you subscribed to for 24 months is already enoght reason not to tie you up for the lock-in period.
The question is, did you agree to be locked in when Globe downgraded your plan back to 2999? If so then that would have been a new agreement. If not, then insist that you should not be locked in since it was not your fault that the speed you subscribed to could not be anymore provided due to their admission as a result of their recent system upgrade.
Just to update:
I've now downgraded back to a 2299 - 5mbps plan without a lock in period.
But now I'm experiencing really slow speeds 1.5mbps or less (using speedtest.net).
I have yet to see the speed even reach the old speed of about 4.5mbps.
If your in the Paranaque/Alabang area and are in need of internet above 5mbps, go to a different ISP.