I was told to wait 1 full day after you change your airline to make a purchase but here is an interesting DP:

I bought one $98 ticket in Southwest and totally forgot to switch my airline to southwest (I had UA). I immediately contacted them to switch my airline to southwest right after the purchase and of course, I canceled it.

After 2 days, my airline incidental fee meter showed $98 spending and I was reimbursed 1~2 days after.