They’re changing references to “precombat main phase” and “postcombat main phase” to “first main phase” and “second main phase,” respectively.

This would’ve inadvertently nerfed a few cards that otherwise were able to take advantage of taking extra main phases via extra combat step cards to trigger their effects multiple times in a turn, like Neheb, the Eternal. Now they’re keeping the old wording on that group of cards so it doesn’t result in any functional changes.