I believe valve suggests not changing the ssd, voids the warranty and all that as well. For me I just got a proper sd card with good stats (A2, class 10, etc.)
That said, I feel like they should be swappable if you were to get your hands on one of the bigger ssds
3.0 Exclusions
The Limited Warranty does not apply if the Hardware:
has been modified or repaired by anyone except a Valve-designated service center and the cause of the issue for which you seek service is the modification or repair, or damage caused by either;