Position eligibility is something that can sometimes be overlooked in keeper and dynasty formats, as most players do not generally lose their eligibility from year to year. However, that slight change can potentially make a large difference for their 2016 value, so let's look at the potential impact. A quick reminder on the rules for the major sites regarding position eligibility:
Yahoo - 10 games in 2015 will guarantee eligibility in 2016
ESPN - 20 games in 2015 will guarantee eligibility in 2016
CBS - 20 games in 2015 will guarantee eligibility in 2016
It Hurts, It Really Hurts
2016 Eligibility: UT, possibly 3B
Losing: SS (0), possibly 3B (9)
Sano qualifies at both third base and shortstop in Yahoo leagues right now, but with 9 games at third this year, he is in danger of being UT only to start the 2016 season in a lot of leagues. He should hopefully get that last appearance at the position before the end of the year to guarantee it in Yahoo. He's still ownable in all formats, but limits his value slightly.
2016 Eligibility: UT
Losing: C (0), OF (11)
Gattis is still hitting for power (23 home runs), but with a .238 batting average, will make him a backup option only in most leagues, and a deep-league play in the best-case scenario.
2016 Eligibility: 1B
Losing: C (0), 3B (0)
Santana's performance this season points to him being a corner-infield option only based on the depth at first base. If his batting average moves up into the .260+ range, the potential for a 15 HR/10 SB season would make him start-worthy at first. He still has a lot more value in OBP leagues as well.
2016 Eligibility: 2B, OF
Losing: SS (0)
Zobrist as a shortstop is now over, as he has not made a single appearance there between Oakland and Kansas City. However, there is a slight chance that he could get third base eligibility before the end of the year (4 games, 3 starts so far)
2016 Eligibility: 3B
Losing: SS (6)
Machado has made 6 starts at shortstop this year while J.J. Hardy has been injured, and he is actually the top player at the position in Yahoo right now. However, he's not likely to appear there more this year, and definitely not enough to get him to 20 games.
2016 Eligibility: UT
Losing: 3B (0)
Talk about a total surprise of a season. Rodriguez has been relevant all year for fantasy owners, but a move to UT will make it just that little bit harder to roster him in 2016.
2016 Eligibility: OF (and gain 1B)
If you owned him this year, you know how much (and how little) fun it was to have him. He got off to a great start, but injuries caused him to miss significant time and a complete ineffectiveness will shift him to first base next year. He'll still have outfield eligibility, but that will only be for next year also.
It Was Nice While It Lasted, But It's Not That Relevant
Miguel Cabrera - Cabrera had 3B eligibility from his 10 appearances in 2014, but that's not happening again this year and doesn't really hurt his value all that much.
Chris Davis - Davis qualified at third base for 2015 due to the time he spent there in 2014 while Manny Machado was on the disabled list. He can't be at the top of the list next year, but he'll still retain his first base and outfield eligibility,
Jose Bautista - Bautista spent time at first base in 2014, but hasn't appeared there at all this year. He'll be outfield only in 2016.
Stephen Vogt - Vogt has not appeared in the outfield this season, but you weren't using him there anyway, so no real loss here.
Mookie Betts - Betts has not played any second base this year, and will be an outfielder only in 2016. This was a nice bonus given his production, but he should still be a starting outfielder in all fantasy formats.
Xander Bogaerts - Betts' teammate played a lot of third base in 2014, but has established himself as the everyday shortstop in Boston and will qualify there only next year. It was nice flexibility to have, but doesn't really change much.
Matt Carpenter - Carpenter earned 2B eligibility with 9 appearances there this season, so he may hold onto it in Yahoo leagues but not necessarily any others.
Matt Duffy - Duffy currently has 2B/3B/SS for Yahoo, but will likely lose 2B and definitely lose SS in all formats. He's appeared in 9 games at 2B this year, along with just 1 at shortstop. He's been a nice find
Justin Turner - Turner has had a great year primarily at third base for the Dodgers, but he qualified at all four infield spots in Yahoo, and will not in 2016. He might get to keep 1B and 2B for Yahoo formats (9 games/6 starts at 1B, 5/3 at 2B), but that's it.
Odubel Herrera - Herrera came into the year with second base and shortstop eligibility, but has played outfield exclusively for the Phillies. He's been a nice option this year, providing a little bit of everything, but will still have value as an OF only.
Right on the Cusp
Kyle Schwarber needs one more game at catcher to reach 20 this year at the position to guarantee his eligibility there for 2016 in all leagues. That should hopefully happen very soon.