there's a september rosters world template that gets released each year, recently... after that, all trades and such aren't done in hindsight for '18 (really "2017", don't let name fool you)
found on mods central... can get there from in-game.. i can't give good instructions as i rarely use it.
if games start on april 1st, you really shouldn't even get a september update unless you plan to start your game in september... it's a simulation, not keeping score of real games.
i hope that stuff is volunteer based... it really makes no sense to put players that were not on 'that' team on april 1st ON that team april 1st. any real resources spent on that would be disappointing. people's free time is their own prerogative, on the other hand... more power to them for doing somethign they enjoy. the business i give money to i hope does not waste my money inefficiently like that
p.s. i'll probably ruin it if possible... if the demo allows export of players, you can use a '19 demo to export new rosters... depending on the year, you may need to do some minor editting to that exported file... definitely need to re-use existing playerIDs in any destination league for import.
when it comes to players, not much changes each year... if something is 'added' in future, just assign 1/2 scale or whatever is rational to all players (like when personality was added, this would essentially be the ~same as when it did not exist -- ie 100/200 personalty scores)