FlyerTalk Evangelist and Ambassador: The British Airways Club
Join Date: Jun 2008
Location: Somewhere between 0 and 13,000 metres high
Programs: AF/KL Life Plat, BA GGL+GfL, ALL Diam, Hilton Diam, Marriott Gold, blablablah, etc
Posts: 33,167
In principle you can change tier whenever you reach the right TPs, mid-itinerary is no issue at all. However, do note that your status only gets updates typically one day after you reach the right TP threshold, and TPs themselves only get credited 1-2 days after you fly and potentially much more on some partners. As such, for instance, you wouldn't immediately be able to get lounge access etc but only at least 2-3 days after you flew.