Need Advice - Identity Point to MGM Tier Credits?

You can't earn MGM TCs or Rewards Points for spend at Cosmo until the switchover occurs, UNLESS you are staying at an MGM property and can charge it to your MGM room.

In any case, signing up for an Identity card and using it when paying would be a better deal anyway. Assuming that you can get Identity TCs and Points for that particular spend.

You'll get 5 cents in Identity Points for each $1 of spend vs.1 cent in MGM Rewards Points

And 5 Cosmo TCs for each $1 of spend vs. 4 MGM TCs.

When they merge, they'll move both to your MGM account at a 1:1 ratio.

There's some speculation they may improve the TC conversion ratio (because 1:1 is a ripoff for gamblers), but that would only make it an even better deal.