As far as I know an early XP is the same saw as a late SE, it just is a matter of when the saw was made - and they all came with a closed transfer (port) cylinder.
That doesn't mean that all SE ones are exactly the same as all XP ones of course, as many changes were made during the production run (1981 to at least 1998, although discontinued on the US market much earlier (and replaced there by the 268XP)).
As far as I know, no important changes were made at the same time as the designation changed - but the venturi size of the carb was increased to 17.5 mm some time in 1985, less than a year before the designation change. This likely was the most important "update" ever done to the model.
Even though the 268XP shared the bore/stroke and belonged to the same "family" as the 266, it really was quite different in many ways - including (but far from excluded to) much larger transfer ports in the cylinder, larger muffler, different case/top cover (including the interface between those parts) etc.
The 66.7 cc "open port" models in this "family" were the 66 (based on the 266) and the "plain" (not SE, XP or Special) 268 that was made from 1991, after the 272XP replaced the 268XP as the "top model" of the family. Production of the 268XP continued for a limited time after this as well, under the 268 Special designation (some had a mix of "XP" and "Special" labels on them).