From my memory:
When beating a Legend in Matchplay, your average may have dropped
from 63.3 to 62.6. -0.7 per win was the margin that I experienced then.
Credit wins are another beast. I didn't experience it, but from reports here, they may not be indicated in average changes but nevertheless result in tier-ups (at a higher ave.).
You should see it after tier-up, before you played your first round. In that state,
new tier and old average are shown. The reset happens after the first new score.
It certainly is possible that the MP win moved my average enough to drop below 63 resulting in the advance to TM. I didn't play very much MP then. It's strange how your memory plays tricks on you, so I looked at my awards section, both CTTH and Stroke play.
The 3rd place Valhalla Challenge Sweeps Skills Challenge shows there and in the 2015 Virtual Tour Tournament Leaderboard (TM) results for that Tournament. It ran the month of August, 2015. I remember playing the last round as a new Legend, but had started it as aTour Master.
My WGT results were a 2nd (tied for first, lost on a tie breaker) for WK 33 free Multiround Best of Par 5, and a 2nd WK 34 Nation Pinehurst 9 hole unlimited play Tournament. I had just hit Level 90 and joined WGT Nation. I remember the timing on that as I purchased the Rsi2 set. Week 33 was August 10-16, and Week 34 was August 17-23, 2015.
The win I was thinking of was actually 2 CC multiround Best of Par 5 Tournament wins. I am the Tournament Director and I went back and looked at the time those 2 took place. Both Finished on August 24, 2015. That was Week 35, August 24-30, 2016. Being Par 5 there was no effect on average.
I remember getting the notice of my winnings for Valhalla Challenge and the Pinehurst Tournament, and then saw that I had been moved up to Legend also. I'm sure my average was 63+, and I think it was 63.3.
That's the best I can reconstruct from memory with the facts available. I remember being surprised about the move to Legend, as i expected some more time as a TM. Knowing it was coming I had started playing practice rounds from the car-park tees. But i wonder how many of us tiered up feeling we were completely ready for the move?