You certainly can, and the improvement is fantastic. What we have is a stand alone cloner which you can purchase cheaply at electronic shops. Clone the hdd onto the ssd and then simply slot the ssd back in. You can even go down to 2.5 inch and double sided tape it in place. You must use an ssd that is as big or bigger than the hdd. If you want to go down in size then you will need to go through all the bother of partitioning the ssd for mac and then putting an operating system on the thing then transferring the data, etc. Much easier to clone then update.
Thank you