It may have been Oct '12 when WGTIcon announced some anti-sandbagging changes in the tiering system, and it has been reported a few times since then.
As usual, WGT doesn't provide details, and I don't know more than rumour stating that
- winning credits in stroke play circumvents the minimum rounds number,
- winning credits in stroke play accelerates the tier-up process,
besides the fact that I experienced
- winning single matches against higher tiers reduces the average considerably.
This is not valid for Legends and TLs.