I've never played in an AL only or NL only league. What happens in your league if someone switches leagues during the season? Is it a free for all, or do you have a free agent/waiver budget?
I recall from reading Fantasyland that they were allowed to retain the player and accrue stats for the season, if they switched leagues. Premise of the book was that the author joined a prominent AL Only league and wrote about the experience and the other players.
In my AL Only league, it's more cutthroat and you don't accrue further stats if he changes leagues midseason. Funniest instance I can remember was the Manny to Dodgers trade, if only because all of us thought Manny had an untradeable contract.
When a new player enters the league, a new bidding process gets initiated. So when Jonathan Lucroy went from Milwaukee to Texas, we had another blind bid for Lucroy in AL Only. Usually the prices get absurd because it's one of the last chances to improve your team; I thinks somebody put in a $40 max bid for Lucroy (our cap is $300).
I grabbed Kelvin Herrera with one of my last picks last year for $1 with the thought that the Royals may move Wade Davis. We're only allotted a handful of keepers, but now I have the option of keeping Herrera for $8 next season which would be several dollars cheaper than he'd go for in an open auction.
It's early, but the other guys I'm considering keeping:
1B Mike Napoli ($8) - Obviously contingent on him signing in the AL. Probably would let him go back into the pool.
1B Greg Bird ($7) - Picked him up late in the season solely as a potential keeper. But not sure if he'd go for much more in our open auction.
2B Devon Travis ($8) - Put up great numbers but seems to have injury concerns.
OF Carlos Gomez ($10) - Decent numbers during his Rangers stint. Went for $31 in the auction this year, then got dropped when he was released by Houston. I picked him up after he was signed by the Rangers. A 20/20 threat might go for $15ish in an open auction.
RP Kelvin Herrera ($8) - I'm gonna be bummed if the Royals bring in a different guy to close.
SP Jose Quintana ($19) - Great season outside of wins. But $19 seems close to full price. Might be rendered moot if he gets to an NL team before April.