Again I need somewhere to put my thoughts to reference later so I don't have to go through the grueling thought process over this again when the time comes.
There's far too many interpretations and expectations with this. It's not explicitly stated that these will replace their base skills, or inherit from. There's far too many different outcomes based on how they implement it:
Kinda sucks, but it would mean deciding on having your Ded give Mentor's XP gain (honestly isn't really used, but there's niche applications) vs a more useful Mentor+. It's a no-brainer for Guardian+ and Starshine+ as there's only plusses. Drifter+ makes the most sense being a replacement as it's fixing the base Drifter's Heal value anyways.
While it does say they change "effects and rates", we already have Mentor go from On Victory to After Moving. It makes sense, as On Victory is a very-passive skill anyways (and we already have skills that provide Special Attack/Defense or movement options), but two active conditions haven't been on one skill, as far as I can remember; because of this, replacement seems likely.
The other likely half. It removes the need to make a permanent decision, and there's zero downside for us, relatively speaking. The logical error comes down to value fixing, be it Drifter's Heal or Guardian and Starshine rates. Would this mean that they, effectively, activate twice? When it comes down to rates, would the second skill activate only if the first skill doesn't? Do they both activate at the same time? When it comes down to the printed rate, is it the final rate (as in, the end value you would get if you were to statistically sample activations)?
Additionally, with Additional Skills being listed in Card#Card Face Translations, it'd make sense that that's what they are: additional skills. Sure, it'd be pants on head retarded to list the same skill twice, but the fact that it has always existed means that it's probably for it,
and not a marriage system.