The speed that's stamped on a 68040 CPU is what it's rated to handle, not the speed it will attempt to run at; that's completely dictated by the logic board. If you put a "25mhz" 68040 in place of a 68LC040 that's running at 33mhz it will run at 33mhz, it just might not be reliable doing it.

Chances are a 25mhz chip will be "okay"; the speed ratings they stamp on the chips are based on very conservative testing and the actual silicon is the same. But there does exist the possibility that the chip won't be reliable at that speed, and if that turns out to be the case your only option will be to buy another one, unless someone here knows how to adjust the clock speed of the LC575 motherboard.

EDIT: Okay, here it says you can clock down the LC 575 motherboard by changing some resistors:

http://www.applefool.com/clockchipping/lc575.html

Note that they're itsy bitsy surface mount things.

The other possibility if it's not reliable might be to try putting a heat sink+fan on the 25mhz chip overclocked to 33mhz.