Quick Rarity/Balance Question

Hey all! Working with my GM to upgrade a +1 weapon with a common magic item effect and trying to figure out if it needs attunement/ change in Rarity. 

We are essentially adding the Horn of Silent Alarm to a +1 Scimitar. Does that upgrade it to attunement? Or to a rare item?

There's no strict formula for rarity, plenty of published items have very questionable rarity. There are suggestions in both the 2014 DMG and the 2024 DMG, but neither one gives remotely solid rules. My answer would be 'no' on attunement and 'maybe' on rarity.... but unless you're using the crafting rules rarity doesn't matter much, and if you're going to be crafting, can easily finesse it to a level not in the rules (say, it costs 2x as much as a normal uncommon item).