This depends on several factors:
- Your age
- The prior condition of your breasts
- Assuming no weight gain/loss in interim
- Hormonal factors
- Skin Elasticity
- Tobacco use
- Submuscular vs Subglandular placement
- Textured vs smooth surface
Without being aware of these factors, as well as other issues, It is very difficult to predict what the shape of your breast will be or the condition of the skin envelope when you downsize.
With that being said, downsizing from 500 to 425 with a different profile should generally not cause a significant amount of skin laxity or breast ptosis (sagging).
This is a relatively modest change and may be barely discernible to you. So I would not be overly concerned about any dramatic difference. In the event that you experience a minor amount of ptosis, a breast lift could be performed at a later date but that will most likely be unnecessary, in my opinion.
During the recovery process, I would encourage you to wear a supportive bra 24x7 to support the mammary parencyhma (breast tissue) and encourage contraction of the loose tissues in their new and smaller position.
I hope this helps!