HELP: Trying to change nature on legal pokemon, and keeping the legality

Hello, I'm having some trouble. I'm using Pokesav HGSS, and I have a level 22 Female Bayleef, that I got from Prof. Elm(at the start of the game).

Here's the problem: I'm trying to change the Bayleef's nature from Quirky to Bold. Problem is when I change the nature to Bold I get a "This is a hacked pokemon" error in the legality checker. I've tried to generate a PID, but it's not giving me a Bold Female, it's only giving me Bold Male. I messed with the IVs alittle(changed it's Sp. Atk from 24 to 20) and did find a Bold Female PID, but when I put that into the checker it said "Never seen this. B-A-C-E [unrestricted]".

If someone could help me out that'd be great. I just want to change it's nature, keep it's gender, and keep it's legality.

Thank you for checking my post, I look forward to a reply.

Play around with your IV values for a bit, then go for the PID/IV button. In the PID/IV Generator screen, you'll want to make sure that not only the PID matches the IV, but the Algorithm must be Type1 as well. Hope this helped!

I need help on changing my Nature as well. The thing is, I have no idea how to do it.

I have a Lugia that I edited the IVs while still keeping the same type, nature, and Abil (I dunno what the Type and Abil mean so if somebody could explain this too that would be great).

Here is an image from the PID/IV Gen:

As you can see, my Lugia is of Type1, Nature Hasty, and Abil 1. What would I do if I wanted to keep it legit/legal/whatever but change the Nature to Modest? I know that I might need to change the IVs a bit but I have no idea on how to change the Nature while still keeping it legit. If somebody could explain this to me that would be great. I have attached the Lugia.pkm file too if it's needed to help explain it, but I would like to know how to do it and keep it legit so I can do it to my other pokemon as well.

Makar's Lugia.pkm