DG isn't like other pro sports where the season ends and then there's a free agency window.
It's probably been confirmed at some point, but for the most part the contracts are thought to land on the calendar year. That's why even if you see an announcement like Matty O, he's not publicly throwing Westside until his old contract expires. And even if USDGC or the DGPT Championship might be considered the end of the touring season, you'll see players hit random sanctioned events during the winter months.
In other words there's not necessarily a formal etiquette on these sponsorship change announcements. Depends on the player, where they're leaving, and where they're going.