Yes it is possible, Yes it is an easy diy job with a 5-point pentalobe screwdriver found many places including ebay.
The lowest cost option is also ebay for a larger drive such as the 256gb or the costly 512
OWC also offers an Aura Pro ssd for the mid 2012
Then sell your 128GB once done to reduce the upgrade cost
NOTE the mid 2011 & 2013 SSD drives are NOT compatible, they look nearly alike yet they will not work