Yes, you will lose whatever you now have if you just replace the hard drive. The option to buy another Tablo probably works fine but you’ll probably never be able to combine the content.
But, just a thought, but you could also use the Tablo Desktop Player to rip your shows you want to keep to your long-term storage, then replace the drive with the new one. At least you won’t completely lose your recordings.
It’d be a cool feature if Tablo would then add some type of an interface to re-add them to the new drive.
Good luck.