If your SSD is large enough to accomodate all data which are currently on the external drive, it is easy enough to clone the drive and then switch the drive letters around so that the SSD has the drive letter which the external drive has before.
Don’t try to move too many things at once. Chances are that you’ll confuse things if you haven’t done the procedure yet.
Btw. there is not really a necessity to put media files onto an SSD.
I’d put the Plex data folder onto an SSD, yes. But for media files it feels like a giant waste.
