Can I simply swap a Time Machine SSD with the original one on an MBPro? I need to replace the original internal SSD (256GB) with a 512GB that has Time Machine on it and up to date.

I am hoping it were not required to involve yet a third disk - e.g. another USB hard drive in this process .. but am not clear on it.  Let us call this 1TBdrive 

Specifically it seems one solution would be:


attach the 1TB hard drive and make it the time machine backup.  
power off the MBPro and physically swap the 256GB with the 512GB
attach the 1TB USB hard drive and boot up, keeping the option button pressed to induce booting to time machine
restore from 1TB TM to the new 512GB internal drive


Unfortunately I do not possess this 1TB external drive.  So is there any way to do this with just the first two SSD's?

UPDATE Here is the relevant available disk space info

df -k

Filesystem                        1024-blocks      Used Available Capacity  iused    ifree %iused  Mounted on
/dev/disk1                          243915264 235014420   8644844    97% 58817603  2161211   96%   /
/dev/disk7s2                        499771696 214002444 285769252    43% 53500609 71442313   43%   /Volumes/os2mellyrn


Notice the externally attached SSD used for TM (disk7s2) still has 280GB available = more than twice the internal (active) SSD (disk1)
As I understand the question, you don't need another disk. But be careful if this is your only backup.

It's possible to clone the current bootable drive (256GB) onto the same drive as your Time Machine Backups (512GB with 280GB available), so that it contains a working system and is bootable.

It should possible without third party tools, but it's definitely possible with them.

Take SuperDuper! for example. From their FAQ:


  How do I store a bootable backup side-by-side with my Time Machine data?
  
  It's actually really simple. Assuming the Time Machine volume is
  properly partitioned and formatted for the Mac (as "GUID" for Intel
  Macs, or "Apple Partition Map" for Power PC), and is boot compatible
  to start with, just use "Backup - all files" with Smart Update to
  store a bootable backup on the drive. Your Time Machine backups will
  be preserved.


CarbonCopyCloner is another example of software which can probably do this, but I've not seen the exact details in their knowledgeable. It could certainly do the cloning, but you'd want to make sure it didn't mess with the backups assuming you have need for them later (moving them off when you get a new larger drive, etc).

But, if you don't need those backups in the future and are prepared to erase them, then you could just format that drive and make a bootable clone of your current system on that drive.

Note: The recovery partition would be a factor here. I'm not sure what, if anything SuperDuper! would do to create one. I know CCC will work with them.

Note 2: If needed you can reconnect your Time Machine with one of the methods explained here.