I have Silestone counter tops in my kitchen that were brought into the house in 2 pieces and then seemed together.
I want to now replace my cabinets. Is there a way to un-seem the Silestone neetly so it can be removed and then put back on the new cabinets and re-seemed?

Your installer can cut the seam but this is not a DIY project. It takes skill and equipment not found in most tool buckets. If you are having a pro install the cabinets he may be able to do the job but not normally. Call a countertop installation outfit.