Ultegra is highly servicable.
Be very careful with the seals. There is usually some press-in metal insert over the bearings. Some people advise removing it, others say leave it in place. I managed to damage an insert by removal, I say leave in place.
When replacing cones, ensure you have the correct model, they vary in thickness and you need a good match to make the seal. You also need the right version of the rubber sealing ring.
Bearings should really be replaced during annual service. Use ISO 25 grade. I find that packing full of grease wil limit the room for dirt to enter.