Would a breast lift allow me to stay my old size (34D), or would I have to get implants?

There is a new technique called The Ultimate
Breast LiftTM. This technique
reshapes your breast tissue creating upper pole fullness, elevates them higher
on the chest wall and more medial to increase your cleavage. This technique avoids the ugly vertical scars
of the traditional technique, maintains nipple sensation and the ability to
breast feed. You will not lose volume
and still remain 34 D. If you are happy
with that size, I would not recommend implants.
Implants are only designed to increase volume, they do not change your
shape.Best Wishes,Gary Horndeski, M.D.

You are describing loss of volume when you say you don't have the fullness you used to have. A lift will reposition the nipple onto a higher location but you will still be hollow on top as you are now. You will need an implant to recover the volume you lost.

The only way to determine this is to examine you in person.  You need to see a BC PS in your area.  I would recommend Dr. Clapson in Saskatoon.