Afaik the crystal is used to precisely generate the clock for the USB-component(s). When you do not need USB capabilities there's nothing against changing the quartz.
USB needs 48 Mhz on PSOC, why 24 Mhz is used X 2.
If you need another frequency consider PSOC 4 which has fractional
clock dividers, or using verilog to generate a fractional divider to gen
the 18.5 Mhz. Note the downside is clock jitter in these techniques.
As well as absolute accuracy due to finite resolution of the technique.
http://www.cypress.com/?id=2401 Discussion half way down the page
Google "verilog fractional clock dividers", you will get a lot of hits.