Well, I'm one of those too. My hair has been *many* colours throughout time. My natural colour is a level 2 very dark brown, that most people call black even though black dye is darker.

Dying my hair has always loosened my curl. That could also be attributed to the fact that dying my hair any other colour besides black is lightening and I've only ever dyed my hair black once. It was back in high school and I was blowdrying/combing my hair straighter back then anyway so I really can't say.

Straight up bleaching really loosens my curl, quite unevenly too. Despite having totally fried my hair with bleach (or too high developer... or both) in the past, it still could never kill all the curl. My 3a/3b turned into more like 3a/2c, but it still looked funky cuz my hair doesn't lay flat easily.

Now my hair and skin is quite sensitive and very easily damaged, more so than others. Despite this it remains quite stubborn to any kind of treatment as well. XD

I've found that with my hair I must do multiple mild treatments to it to get it to do what I want with *minimal* damage. Bombing my hair with one big shot always did nothing but fry my hair and scalp.

When bleaching I did get my hair to a nice light med blonde, but this was only after a year of slowly bleaching and toning my hair up to that point. I can't use anything higher than a 20 volume developer and I never left the bleach on longer than 30 min cuz anything longer than that... well the only thing that fixes that is a pair of scissors. In the end, this still ended badly as I one time left the bleach in for 45 min. I tried for a month to save my hair with no luck.

Even just trying to dye my hair so my grays don't show was a pain till I tried Wella Colour Charm. Friggin' grays are so resistant to dyes, so many of them just wash out, To top it all of the Wella even works with only a 10 volume developer. Aah! my hair is so thankful!

These are my experiences, and they probably won't resemble yours. =) I'd say overall going lighter will probably cause some loosening with it's effect highly variable depending on method and hair condition. Not sure about darkening.

On that note I do would like to echo Korkscrew: Even relaxing my hair doesn't kill my hair as bad as bleaching it does! Yikes!
Very fine, high porosity, currently dyed and growing out the relaxed portions. I'm pretty sure my hair's 3b something IIRC. We will be finding out for sure as time goes by.