Or you can use upgrades, but in this case you won't be able to reverse process.
If you want to have ability or whatever that will increase hp or reduce it when you pick or drop item, kill unit, cast spell I suggest Maker solution. (Single unit)
But if you need it for some defense like map or map where you increase unit's hp permanently, I suggest player upgrade (it will increase hp for whole class of units, for example all player owned footmains). (Group of units)
Also if you want to increase hp of all units owned by some player, use handicap option, it's value is set to 100% by default, so changing it to 200% will double units max hp and so on. (All player owned units)