To start with you are not moving the OS, you'll be installing a fresh copy on the SSD from your Lion or Mountain Lion installer.
As long as you boot up under your FileVault drive using your user account the files were saved under you can then copy them over to your SSD.
As the SSD does not have FileVault running the files copied over will no longer be encrypted. Think of it this way, if you plug in a plain old USB thumb drive and copy a file to it any then tried accessing it on a different system you would not have any problems access the file (not protected), this is no different.
The rub here is when you flip the boot drive over to the SSD. Now your FileVault drive won't be accessible as the key chain on it won't have the needed keys. Here's a good writeup Apple TN on FileVault and the one MacWorld writeup