Why should crystal switching be avoided?
What issues might I expect to encounter if I were to attempt crystal switching such as in the schematic below?


The VCO will run at an incorrect frequency, and is likely to fail to lock either video standard.


  Are there any measures I might take to mitigate these effects?


Use a different part -- preferably one that's still in production. This part would have transitioned from Motorola to On Semi when Motorola's analog division was spun off in 1999, but On's web site doesn't even acknowledge that the part ever existed.

If that weren't the case, or if a similar issue arose with a substitute part, I would recommend using two sync separators, each with their own crystal, and switching between their outputs.