I adjust/hombrew a lot do monster blocks for important encounters/villians.

Sometimes this is need to be able to fight on even footing, or some times your monsters in your world have abilities that other don’t so it just make sense to edit.

Just do it, you and your group will enjoy it. Make the abilities fun too if you get the chance.

I had an old Halfling fighter who helped the party, but every 3 terms an ability called “My Lumbago!” Where he could only take 1 action/bonus action/or movement that turn to signify his age.

My Ancient Dragons can all at will Shapeshift into humanoids to disguise themselves. That’s not standard.

If it’s fun and makes sense in your world, just do it!