Can I simply swap a Time Machine SSD with the original one on an MBPro?
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.