If I lose weight after my breast reduction, a significant amount like 60-70 lbs, will my newly reduced breasts lose their shape?

Losing a significant amount of weight as you stated can always change the shape and contour of a breast reduction.  I always ask my patients to lose the weight prior to surgery.

Although
it’s impossible to predict exactly how weight loss will affect your figure,
most women do notice that their breasts lose a fair amount of volume in the
process. If you have a breast reduction before your weight loss, it’s possible
that your breasts will reduce further and a revision may be needed. On the
other hand, if you delay your reduction procedure until after you’ve reached
your target weight, there may not be enough breast tissue to qualify for
insurance coverage. Depending on your personal preferences, I like to recommend
going with one surgery whenever possible rather than scheduling a surgery right
now that could mean needing a revision later on, but you need to make the
choice that’s right for you.

For most women, fluctuations in weight mean a change in breast volume and appearance.  If you lose a significant amount of weight after breast reduction then its possible that you will need revision surgery to address any loss of volume or breast sag.  You may require a breast lift or implants to help restore your volume.  Good luck.

Losing a large amount of weight such as 60-70 lbs after a reduction definitely puts you at risk of having your breast appear more saggy and flat on top. The reason for this is that the breast is comprised of a great deal of fatty tissue in the typical patient, and weight loss will cause this fat to shrink in size. If/when you undergo your reduction first, the skin will be a good fit for the amount of breast tissue present after the surgery, but if the amount of tissue decreases then there will be too much skin, thus a "saggy" appearance. While there is always the chance that your skin may have some elasticity and tighten up some with your weight loss, a lot of people have a very deflated looking upper breast area after losing weight. The best thing to do may be to see how this fits into your overall long term plan, and proceed accordingly - don't be in a rush to have surgery if the results will not be ideal.

Breasts are part fat, so if you loose a lot of weight, your breast will change shape.If you have a reduction now, your plastic surgeon will make your breasts the best for your weight now, when you loose weight you will also loose volume, so I am afraid the result will be lost.Best of luck 

Many, but not all, women that lose that amount of weight will see a significant deflation of their breast to the point there is not enough weight to remove from the breasts to get insurance coverage. But if you do the reduction first and lose that amount of weight, it is likely you will need augmentation later on. Your choice in what comes first.