I read voraciously on all 31 teams, while continuing to watch film on prospects. Draft eligibles are like stock in the sense that they rise and fall for certain reasons leading up to the draft.
For instance, Casey Mittelstadt had the talent to be a top-3 pick last year. But a poor showing at the scouting combine dropped him in public estimation, and he wound up going 8th overall to Buffalo. Who went 7th but Lias Andersson -- a very solid, two-way middle six LW/C prospect, certainly, but the talent discrepancy between Mittelstadt and Andersson is comparable to the talent discrepancy between, say, Patrick Kane and Josh Bailey.
This year, Jesperi Kotkaniemi was widely considered to be a pick in the mid-teens, but he was absolutely stunning at the IIHF. There is now talk of him being a possible top 5 pick.
Teams also change priorities. If the Islanders get close to re-signing Tavares, for example, there is a greater chance of them trading one of their two first round picks in order to get Johnny T some help now. Or Ottawa -- many pundits have them taking Tkachuk at #4 overall, but if they decide to trade Karlsson it is possible they go in the direction of Adam Boqvist, a similarly-styled player to Karlsson at the same RD position, with the fourth pick. Ottawa also has the #22 pick -- if they take a F at #4, they are most likely (judging on the history of teams with multiple first rounders) to take a D at #22, and vise versa.
So, I suppose you can say that a lot can still change between late May and late June, when the draft is actually held.