Guild Wars 2

Yes, you can use those skills normally but if you want to change them you need to be out of combat(obviously) and pick up a bundle so the slots behave like they did prior to the patch.

Now upon reaching lvl 24, my second slot(that already had a skill in it) simply starting working as intended.

Since the 9/9 feature patch update the level requirements for unlocking utility skills slots rose to 13, 24, and 25.

However when using elixir of Heroes( and other bundle items) you can assign skills to the slots that you would already have unlocked prior to the update.

I.e. with a lvl 23 warrior you should have access to only 1 slot now, when before the update you would have access to all 3. By picking up elixir of heroes (and other bundles) and then assigning the skills to these slots you lock them in place, not being able to change them after the bundle is dropped, unless it is picked up back again.
Reloging doesn’t appear to reset the skill slots , which means that the skills chosen remain.