For the record, average here means the average of your X best ever rounds. X depends on your tier. For Tour Master, X = 50 (thus your average is the average of your 50 best ever scores you posted). For Legend, before the last update, X = 400.
Concerning sandbagging, yes that's a possible issue, but not really for the current top players. There is the timing problem mentioned by Javier, namely not all the top players will be promoted at the same time, it depends how fast they play 100 rounds. Those who are playing a lot will be promoted soon, those who are playing less will be promoted a bit later. But that's just a transitionary state and this is not a problem on the long term.
Indeed, a player with an average of 54 (or 57) before the update would need to score an average larger or equal to 84 (or 72) to get an average above 60 during these 100 additional rounds to stay on the long term in the legend tier. This would be too obvious not to be seen.
So beside the timing problem (not all top players will move at the same time), the real problem could be about the next top players to come (who are at a lower tier for the moment or not yet registered). But really if you want to get easy money, it's better to sandbag at lower tier than legend. I don't think it will be a real problem. We will see.
Don't forget also that WGT has said that they will take into account tournament results for tier promotion. We don't know yet how it will be done, but to win tourney you need to post low scores which lower your average. If in addition, wining tourney (or being in the top of the leaderboards) accelerate the process, it will become much harder to sandbag and you will be promoted rather quickly.
Of course there is always the problem with multiple account, where you can start over as much as you want (as long as you go unnoticed), but that's an independant problem not linked to the creation of this new tier.
Regards