Getting new pokemon level before capture

I suppose when u were relogging previous gastly disappeared and a new one spawned. Never had a practise of changing cp of the same pokemon.

Actually, there is a way to do exactly what you want, but it is quite impractical to use at will. All you have to do is this:

1) First, encounter the pokemon you want to catch.
2) If you don't like its CP, flee from the pokemon.
3) Level up before the Pokemon despawn.
4) Re-encounter the pokemon and you'll find that the pokemon will likely have a different CP.

Here's the explanation. As everyone knows by now, wild pokemon's levels are dependent on your own trainer's level. As documented on this site recently (https://pokemongo.gamepress.gg/wild-pokemon-levels), when encountering any pokemon, all players have a 1 in "n" chance of getting a pokemon of a particular level, where "n" is the trainer level, randomly.

Since CP is affected by pokemon level, this is the reason why when two players try to catch the same pokemon in the same encounter, they might find that their pokemons have different CP.

In this scenario, when you leveled up after fleeing from the pokemon, your level changes, and thus, the random pokemon level of the pokemon will be recalculated when you encounter it again.

However, as it is random, you could get the pokemon at a lower CP/level than the first encounter, and it is close to impossible to level up again and have the pokemon level recalculated before the pokemon despawn. That's what I meant when I said that this trick is highly impractical in quick succession.

Finally, do note that I have no idea if this trick will still work if the player is already at level 28 or higher. The level cap for wild pokemons is at level 30, and so there will always be a 1 in 30 chance of getting a pokemon at any pokemon level if your trainer level is higher than 28 (the max pokemon level you can encounter is 1.5 higher than your trainer level, i.e trainer level 28.5 -> pokemon level 30), therefore I am unsure if the random level of the pokemon will be recalculated if you level up beyond level 28 in your 2nd encounter of the same pokemon.

One way that I know works is that if you are just about to level up, you can check the CP. If you are not happy, you can level up and the pokemons level will get randomized again.

As you can understand, the practical usage for this is very limited.