If you have a second class Interrail or Eurail pass, you might want to upgrade to first using something like Seatfrog, Weekend First or (to Standard Premium, rather than First) an onboard upgrade with Avanti.
Or contact Eurail to exchange your pass.
If none of those is suitable, can a passholder excess their pass up to first on a per-journey basis?
Or contact Eurail to exchange your pass.
If none of those is suitable, can a passholder excess their pass up to first on a per-journey basis?