Maybe, but probably not
I believe some of the possible scenarios can fall under this existing discussion, which references two relevant clauses (assuming none of the previous rules in that answer firmly place the question on-topic):
If you're not sure it's SF but you think a good case can be made for it, it's on-topic.
If there is a minor supernatural element (e.g. a fortune teller's prediction comes true, or someone sees a ghost, or a story for children involving anthropomorphic animals) but it's just a throwaway plot element that's not particularly relevant to the question, it's off-topic.
- The earliest Tarzan novels contain zero fantasy or sci-fi content. Latter novels had him shrinking to the size of a pea, fighting ant-men in a volcano and battling Medieval knights at the centre of the Earth.
Granted, it has been a very long time since I read the originals, but I recall Tarzan being able to communicate with animals far beyond what is considered normal, even among naturalists who have been accepted into wild animal groups, developing a sense of smell far superior to any other human, and being so strong one could argue some of the feats he performed were borderline "supernatural". I think those characteristics fall under rule #5, being a good case for it being SF, even if we're not sure.
However, more in line with the gist of this question, the hollow-earth plotlines and other supernatural elements sound like they form more of a background universe within which Tarzan takes place. If that is an accurate description, then I'd say yes, later stories tip the balance from "not sure if it's SF" to "yes, the universe is SF".
- The first four seasons of the TV show Felicity have zero fantasy
content. The last few episodes of the fourth season revolve around
an extensive magical time-travel plot.
Again, I've never seen this, but it sounds like this falls under rule #6: if the question is directly related to the time travel, then it is on-topic; otherwise, the time travel becomes just a throwaway plot element that's not particularly relevant to the question.
As for the other two... I'd say no, later episodes don't automatically retcon something that is unambiguously non-SF into an on-topic franchise. Questions about the earlier, non-SF portions of the franchise would have to directly relate to a later SF element in order to be on topic.
Seasons two and on (did they make more than 2 seasons?) of Baywatch Nights are on topic; Season one is not, unless it somehow directly ties in with a fantastical element of Season two (e.g. "we found out that lifeguard Janice was secretly a were-chicken in episode 5 of season 2; is there any indication from earlier episodes that might have been hints at her condition?" might be on-topic).