It all depends on whether they "recalibrate" the new program to Marriott points or SPG points (or something different than both !). If they leave Marriott points as they are, I expect most Marriott properties could remain the same, but they'd have to "re-calibrate" all the ex-SPG properties since they were on a different points system. If they adopt SPG's point system (less likely IMHO), then they'd have "re-calibrate" every Marriott property.

So if the points system stays Marriott-style, then I expect the fewest changes to properties which were always Marriott, but unpredictable changes to properties that came from SPG.

But meanwhile remember that properties get re-evaluated for points cost annually anyway, merger or no merger.