This guide to resolve any issues users get when they try to change their stats of a Pokémon from Gen 8. DO NOT USE ANY SIGNIFICANTLY HACKED POKéMON ONLINE!
Foreword
One reason why people get various PID-Mismatch or Invalid Encounter type of messages for Pokémon that originate from SWSH, is because they did not modify the Pokémon correctly.
(Remember: if your encounter details are invalid, then doing this won't work. This assumes your other encounter details are valid.)
In Generations 8, due to an in-depth understanding of how the game mechanics call stats for (mostly) Overworld Pokémon, the Personality Value (PID) is tied to various other values related to the Pokémon generation procedure in-game, such as Individual Values (IVs), Height, Weight, and Encryption constant.
This method of correlation is known as overworld8.
You can read more about it here: https://projectpokemon.org/home/forums/topic/58824-swsh-overworld8-pid-type/
Basic Preparation
1. Gen 8 OW PID Genner will be the tool we use to find correct information (PID/IVs etc)
2. PKHeX Plugin for OVerworld8 IV Searcher for searching by IVs (it would have similar UI as the one above, for the most part)
3. Time and patience. I cannot stress this enough.
4. Knowing whether your encounter can have any guaranteed IVs.
5. (Optional) For beginners, I recommend Searching for particular IVs, then Shinification (Keeps PID, changes TID/SID).
Other options for beginners: If you want to search for IVs
I recommend Searching for particular IVs. If you want to make any given legal overworld8 PID shiny
I recommend Shinification (Keeps PID, changes TID/SID). If you don't care about IVs, and just want a shiny with your TID/SID combo
I recommend Shinification (Keeps TID/SID, changes PID). If you want a Pokémon with a certain PID, TID and SID to be shiny
I recommend Shinification (Keeps TID, SID, and PID). If you've become accustomed to how the tool works
you can play around with other options, but all at your own risk. Searching for particular IVs
Shinification (Keeps PID, changes TID/SID)
Shinification (Keeps TID/SID, changes PID)
Shinification (Keeps TID, SID, and PID)