Can I switch in a new storage drive without again downloading the Mountain Lion installer?
Buy an enclosure kit from OWC or another vendor.
Buy and put your SSD in the enclosure.
Connect SSD/enclosure to your computer.
Format the SSD with Apple's Disk Utility.
Use SuperDuper! to copy your entire internal HD onto the SSD.
Set startup to the SSD and boot from it to make sure it boots.
Remove SSD from enclosure and install in MacBook Pro.
Put MacBook Pro's internal hard disk in now empty enclosure and use it as a backup.


You also can use Disk Utility to image the drive if you don't want to use a third party product like SuperDuper! or CarbonCopyCloner. Some people also prefer a simple USB to SATA connector and you can usually get them for less than $5 from Amazon if you don't need or want a drive enclosure for your old drive.

However, the use of your old hard disk in an enclosure and a backup tool like SuperDuper! or CCC gives you both an external drive and the beginning of a solid backup solution as well.