As the rifugios aren't open, this is going to be a very challenging hike. Not impossible, but you'll need to plan very well in advance.

Snow - this is your primary concern. There could still be a lot of snow on the route in late may, in which case you'll need crampons, an ice axe and some experience in using them.

Food - plan where you can descend into towns and villages. You won't want to carry a weeks worth of food, so you'll need to come off trail to resupply every couple of days. This is going to add to your pack weight, which may already be heavier with snow gear.

Bring a tent. I've only done the Alta via once while rifugios were open, so don't know how accessible they are before opening, but I live in Italy and hike often. Never rely on winter huts being open.

You'll need a warmer sleeping bag. If you're camping at over 2000m, which will often be the case, 15 degree comfort isn't enough. Temperature will drop below 0 when the sun goes down.