You can't set a tech level (upgrade) to a level lower than it already is. There is no going backwards.
Also, any suggestion which uses a 1 second periodic trigger is going to kill map performance. You should seriously consider an alternate approach.
Original poster, you didn't specify if this is a unit or hero ability, and you didn't say if it's a passive permanent ability or something which is actively used and wears off after a time.
If it's for one unit and permanent passive, use the upgrade approach. Add levels to the upgrade whenever the hero learns another level of the ability.
Otherwise, your best approach is to create item abilities that do the effects you want, and remove/add them when the hero uses the ability.
You could use dummy caster units with modified Bloodlust for attack speed and movement speed, and dummy casted Inner Fire for damage, but I can't think of an ability besides Avatar that increases life (and that adds in magic immunity).