I want to downsize my new 425cc implants to 300cc. I have perky breasts but thin skin. Is this an easy procedure?

If you have no capsular contraction, this should be a very easy procedure. With a capsular contracture (encapsulation), which you could develop over a period of time, it takes more surgical time, but for experienced plastic surgeons, not a complicated procedure.
The wrinkling that you have makes me think you have saline implants. I recommend silicone Memory gel implants to reduce this risk.
Thank you for your question and photos, and good luck.

It's unfortunate you aren't fully satisfied so soon after your initial surgery.  I am inferring these are saline implants on top of your muscle.  If you want to be smaller and have less rippling, downsizing to gel is a good idea and a plane switch (to under muscle) should be considered.  This is best for an in-person consultation.  I would advise a consult with a practice that likes secondary breast cases, and plan on saving for a re-augmentation to gel with lift.  If a lesser procedure is acceptable for you, all you would have done is over-saved, which is a good thing!

When downsizing implants, the skin has to shrink a bit or there will be sagging. One thing you can do to help it is to have the implants intentionally deflated. It's an in-office procedure, and then over the next few weeks the pocket and skin will shrink. The new implants should be a higher cohesivity type such as Sientra of Allergan Soft Touch, and the base diameter and profile will be important to minimize rippling. Some patients need Strattice or Galaflex to add coverage.