The answer really depends on the type of rating.
Ratings during or affecting Championships
Changes to a player's rating during Championships take effect at the end of Championships. This is an exception to the rules below.
New Ratings (for unrated players)
As an unrated player, you typically receive a rating during your first tournament. That rating is effective immediately,
E.g., if you get rated BB on your first tournament, you can only play BB or higher going forward (until your rating changes).
"Uprates" (for previously rated players)
If you enter a tournament as a rated player and, during that tournament, you are "uprated" (re-rated to a higher division), you actually keep the old, lower rating for 35 days.
E.g., if you were BB and were uprated to A on March 5, you can still play BB for the following 35 days (until April 9). On and after April 10, you'd only be able to play A or higher.
"Downrates" (for previously rated players)
If you enter a tournament as a rated player and, during that tournament, you are "downrated" (re-rated to a lower division), the new, lower rating is effective immediately.
E.g., if you were BB and were downrated to B on March 5, you can play B at your next tournament.