how does heighten spell work

[Atalius]

I'm a bit confused how this would work. Lets say I have Heighten Spell. I prepare mirror image 3x and 1x Shackle. I use heighten to raise the level of Shackle by +1 thus costing me a level three spell slot. Now what happens with my level 2 Shackle? Do I still have that level 2 Shackle but just lose a third level spell of my choice? Could I cast Shackle again with Heighten and sacrifice another level 3 spell of my choosing? or would I lose both the Shackle and the level three slot (I highly doubt this)?

[*Thelith]

If you are a prepared caster you have to prepare it as heightened. So it would take up a level 3 slot when you prepare it. So you could prepare a normal shackle in lvl 2 slot and a heightened in a level 3 spot.

If you're not a prepared caster you just use up a slot of whatever level it ends up being after heighten.