Had some trouble moving a TV modulator to operate on the old channel 1 frequency, 44.25 picture carrier, or 47.75 if I wanted to inject it directly into a modern 41-48 MHz video IF strip. From a huge stash of old police radio xtals the closest frequencies I could find was 45.3 for channel 1 (only 50KHz off) and 45.638MHz (112Khz off, a little more problematic in that there'd be no fine tuning to dial it in to fit the IF strip filter better).
Before, the modulator osc circuit wouldn't run at frequencies this low, but I increased the loading caps (caps from the xtal to ground). From about 4pF to 8pF. Now it runs. Seems that lower frequency xtals need bigger loading caps. Likely to yield a similar reactance at the new freq like that with the old cap at the old freq. Those of you who have experience in xtal osc circuits would recognize this, probably better than I would. Some hard core engineering info:
http://www.analogzone.com/hft_0102.pdf
Figs 5 and 6 show that larger caps shift the osc freq range down. Crudely speaking, this would make a circuit that worked at a higher xtal frequency work at a lower freq xtal.