Tokøta from Mastering.com breaks down how Apple Loops handle tempo and key changes inside Logic Pro. Region Inspector's Follow Tempo option snaps loops to your project grid automatically, or you can uncheck it and manually stretch a region with Option-drag for finer control.
Pushing time-stretch too far introduces audible artifacts, something worth listening for before committing to a drastic tempo shift.
Changing the project key also updates every melodic Apple Loop to match, so setting your key early lets you audition loops that are already in tune and in time with your track.