I've changed substrates several times in different tanks and never crashed my cycle.
When switching substrates in my 10 gallon, I just floated the filter media in a bucket of the old tank water while I worked to scoop out the old gravel and replace it with a different color. Once the new gravel was rinsed and put into the tank, I just put the old filter media back in the filter. I tested my water every day to every two days to make sure I wasn't experiencing a mini-cycle but everything remained stable.
When I switched from black gravel to natural colored gravel (brown) and then eventually back to black again (UGH, I'm so indecisive!) in my 75 gallon tank I did the same thing except I kept the HOB filters running in a bucket of the tank water and just put them back on the tank when I was done. I kept the filters running because it took much, much longer to change the gravel in the bigger tank than in the 10 gallon. Again, after closely monitoring for a crashed cycle, everything remained stable in this tank also.
From my experience, if your tank is cycled, you can safely switch substrates if you keep your old filter media wet while you're making the switch. If you want to be extra cautious, you can put some of the old blue gravel in a mesh bag or a knee-hi panty hose, tie off the end, and lay it on top of the new substrate near your filter and this will help to populate the beneficial bacteria into the new substrate.
Switching out the substrate seems like a daunting task, but it's not so bad and you'll be happier in the end if you have a substrate you find more pleasing - unless you're like me and decide the first color looked better after all!