Both Java and Bedrock
If you need to detect when and if a score of a player has changed, there are two ways to go about this. This is generally most useful if you’re using a scoreboard objective that changes because of something the player is doing (e.g. clicking, attacking, sneaking) or the game is doing to the player (e.g. taking damage, change of health and hunger).
If you are changing the score yourself, it is generally advised to also do the effects to the player yourself, as you’re changing the score anyways (which is currently the case in 100% of the time in Bedrock), but in some edge cases this can still be useful to do anyways.
This is the more commonly used method, because in many use cases it’s not important to know the total score, just that the score changed. It is generally used for item click detection, player deaths in minigames and more and involves resetting the score back to 0 once it has been changed.
Some objective types are read-only, such as Health or Hunger. Those won’t work for this method.
So lets assume you have a score that detects a players death and want to do something with that player everytime they die once.
execute as @a[scores={deaths=1..}] run say I died :(
scoreboard players set @a[scores={dead=1..}] deaths 0
You can either set the score back to 0, or reset the score, whichever one fits your system better. In most cases either one will work fine.
Sometimes you have an objective you don’t want to or cannot change. In those cases it’s more difficult to know when an objective changed from the last tick to the current one and involves a second, type dummy objective which we use to store the previous value to be able to compare them. Assuming you’re running this as the entity you want to check, have the objective called deaths and the additional one called tmp, the commands could look like this:
execute unless score @s tmp = @s deaths run say My score has changed in the last tick!
scoreboard players operation @s tmp = @s deaths
This also allows us to know if the scoreboard value has incremented or decreased, here is an example using health
# Command blocks
execute as @a unless score @s health = @s health.tmp run say My health has changed!
execute as @a if score @s health < @s health.tmp run say My health has decreased!
execute as @a if score @s health > @s health.tmp run say My health has increased!
execute as @a run scoreboard players operation @s health.tmp = @s health
The first command detects if the health is not the same as the value in the last tick and the second and third command detects if the value incremented or decreased. The last command updates the health.tpm scoreboard to be the same as the current health.