Positional eligibility is rarely (if ever) rescinded by Yahoo after the season opens. Many players will have eligibility added throughout October and November though.
In the Seguin's case I think this year will be different than last. He gained RW because Dallas tried Benn at centre and Seguin being his wing...now that Pavelski is on the team I can see Benn and Pavelski together or Pavelski as Seguins wing.. Pavelski centering and Seguin being his wing is another posibility but not that likely I guess.