Capacitor dielectric: Is X7R a direct upgrade over 2C1?
You are correct to be suspicious. The voltage coefficient of 2R1/X7R is not guaranteed. It could lose 80% (or more) of its nominal value at rated voltage. You would have to check the detailed data from the manufacturer for typical values. 

The worst-case voltage coefficient of 2C1 is guaranteed, so 2C1 is better (but probably larger/more expensive for the same voltage rating). 

See this reference. In this context, VCC stands for Voltage Coefficient of Capacitance. 


  Greater than 90% loss of capacitance at rated
  voltage is not uncommon in the general market place. But this
  can be avoided for some parts by specifying 2C1 (BZ) or 2X1
  (BX) dielectrics rather than standard commercial X7R (2R1).
  These options have a more tightly controlled VCC at the
  expense of absolute capacitance value in a given package (see
  Figure 1 again) – although, observe that a worst-case
  capacitance drop of 25-30% is still to be expected