Nice idea, assuming they are sufficiently rare to not be overpowered.

This is a very interesting idea, I'm behind it.

I like it too. Making the rarity (and writing difficulty) on par with scrolls of defense and protection would be fine.

What about items with several bonuses (whips of the snake)?

I guess you will have to introduce weights to bonuses - something that will determine the importance and modifiable range for those attribute bonuses.
That whip of snake has both Dx and speed modifiers so I think they should be both upgradable but separately per scroll read - with a 50/50 chance of either being the upgrade target.

You could just have it only apply to stat modifiers and not to speed. No item can have two different stat modifiers.

Alternately, yeah, just have it choose randomly between speed and attribute.

I'm asking because I have a feeling that there is 1 attribute code-wise, as whips of the snake Dx and Speed bonus are always in sync.

Could be. In ADOMBot you can adjust them separately, IIRC, but all of the in-game examples seem to have matching stat/speed. Curious.

There's only a few items that this would be an issue for anyway. I think it's just whip of snake and moloch armor. I guess in principle there's no harm in it affecting both if it has to. These aren't exactly common or regularly used items anyway.

Not that I dislike the idea (more nice loot? Yay!), but the way it is proposed here, unspoiled players would have it very very difficult to figure out what the item does.

They would see "scroll of enchantment" and try it with any weapon/armor (as in Brogue), and it would always fail unless they lucked out and tried it with one of the < 1% of the weapons/armor that have stat boni.

I don't want to vote no but the idea needs some polishing to avoid this... maybe a more specific name, maybe an effect for other kinds of items too, but something.

Maybe just "scroll of increase enchantment value"? We already have scrolls with long names, like increase melee accuracy, this would make it both fitting from the game standpoint and more self-explanatory as to its function.

For naming, I recommend "Scroll of Enhancement"
Would allow #5513 with "Scroll of Enchantment"

Still, backed, would be interesting and useful.