Now that we have the table of contents from XGtE, we can confirm that there are subclasses in XGtE that are reprinted from earlier books. A reminder from p.5 of the current FAQ:
So if you have a swashbuckler character, and thus have SCAG as your +1, you will need to follow the rules for swashbuckler in XGtE, even if you don't own the book. This does not change your +1 from SCAG to XGtE.
(Note: as per Jeremy Crawford, there is at least one change to the Swashbuckler in XGtE -- the Rakish Audacity ability is updated to reflect that a Swashbuckler cannot use its Sneak Attack condition when he has Disadvantage.)
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Pauper
The most current version of any rule is used -- even if it's reprinted in another book. This does not, however, affect your choice of +1.
So if you have a swashbuckler character, and thus have SCAG as your +1, you will need to follow the rules for swashbuckler in XGtE, even if you don't own the book. This does not change your +1 from SCAG to XGtE.
(Note: as per Jeremy Crawford, there is at least one change to the Swashbuckler in XGtE -- the Rakish Audacity ability is updated to reflect that a Swashbuckler cannot use its Sneak Attack condition when he has Disadvantage.)
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Pauper